st/dri: Support 24bit formats in dri2_allocate_buffer

Prior commit 57616128,
the parameter format was bpp, thus both 24bit and 32bit formats were
requested with format set to 32. Handle 24bit seperately now.

Fixes RGBX formats in wayland platform for egl_dri2 (EGL_ALPHA_SIZE=0).

Note: This is a candidate for the 8.0 branch.

src/gallium/state_trackers/dri/drm/dri2.c

@@ -316,6 +316,9 @@ dri2_allocate_buffer(__DRIscreen *sPriv,
 
    switch (format) {
       case 32:
+         pf = PIPE_FORMAT_B8G8R8A8_UNORM;
+         break;
+      case 24:
          pf = PIPE_FORMAT_B8G8R8X8_UNORM;
          break;
       case 16: