Don't crash X server if GPU acceleration is not available

Commit d1d8e3c8 causes X server to fail on startup when GPU acceleration is not working (or is disabled). The reason is that radeon_get_pixmap_bo function gets called too early (before EXA has been initialized) and fails with an assert:

 #0  __GI_raise (sig=<optimized out>) at ../sysdeps/unix/sysv/linux/raise.c:50
 #1  0x76ab1c6c in __GI_abort () at abort.c:79
 #2  0x76ac0b64 in __assert_fail_base (fmt=0x76bfbce4 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x7658c80c "key->initialized", file=<optimized out>, line=121,
     function=0x7658d040 <__PRETTY_FUNCTION__.10607> "dixGetPrivateAddr") at assert.c:92
 #3  0x76ac0c0c in __GI___assert_fail (assertion=0x7658c80c "key->initialized", file=0x7658c9d0 "../include/privates.h", line=121,
     function=0x7658d040 <__PRETTY_FUNCTION__.10607> "dixGetPrivateAddr") at assert.c:101
 #4  0x76579e6c in dixGetPrivateAddr (key=<optimized out>, key=<optimized out>, privates=<optimized out>) at ../include/privates.h:121
 #5  0x7657a954 in dixGetPrivateAddr (key=<optimized out>, key=<optimized out>, privates=<optimized out>) at exa.c:70
 #6  dixGetPrivate (key=<optimized out>, privates=<optimized out>) at ../include/privates.h:136
 #7  exaGetPixmapDriverPrivate (pPix=<optimized out>) at exa.c:68
 #8  0x7623d460 in radeon_get_pixmap_bo (pPix=0x71c1b8) at radeon.h:804
 #9  radeon_get_pixmap_handle (pixmap=0x71c1b8, handle=0x7fa22328) at radeon_bo_helper.c:357
 #10 0x76244458 in radeon_pixmap_get_fb (pix=0x71c1b8) at radeon.h:886
 #11 drmmode_set_mode_major (crtc=0x691860, mode=0x69191c, rotation=<optimized out>, x=<optimized out>, y=<optimized out>) at drmmode_display.c:918
 #12 0x762467e8 in drmmode_set_desired_modes (pScrn=0x67c678, drmmode=<optimized out>, set_hw=1) at drmmode_display.c:3128
 #13 0x0047bfa4 in MapWindow (client=0x669ec8, pWin=0x7206c0) at window.c:2722
 #14 MapWindow (pWin=0x7206c0, client=0x669ec8) at window.c:2665
 #15 0x00449650 in dix_main (argc=3, argv=0x7fa22604, envp=<optimized out>) at main.c:247
 #16 0x76ab2198 in __libc_start_main (main=0x42db10 <main>, argc=3, argv=0x7fa22604, init=<optimized out>, fini=0x606434 <__libc_csu_fini>, rtld_fini=0x77229930 <_dl_fini>,
     stack_end=0x7fa225e0) at libc-start.c:308
 #17 0x0042db80 in __start () at ../sysdeps/mips/start.S:110

Don't call exaGetPixmapDriverPrivate if the acceleration (EXA) is not enabled [yet] to avoid the problem.

Closes: #188 (closed)

Closes: https://bugzilla.altlinux.org/show_bug.cgi?id=37539