fb: Remove 24bpp support (v3)

v2:
- Require power-of-two bpp in ScreenInit
- Eliminate fbCreatePixmapBpp

v3
- Squash in the exa and glamor changes so we can remove pRotatedPixmap
  in the same stroke.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>