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.