There's really no reason to pretend to support this, apps hate it, all
we're doing is giving people a way to injure themselves. It doesn't work
anyway with any Radeon, any NVIDIA chip, or any Intel chip since i810.
Rip out all the logic for handling 24bpp pixmaps and framebuffers, and
silently ignore the old options that would ask for it.

The cirrus alpine driver has been updated to default to 16bpp, and both
it and the i810 driver can now use the 32->24 conversion code in shadow
if they want. All other drivers support 32bpp. Configurations that
explicitly request 24bpp in order to fit in VRAM will be broken now
though.

v2: Fix command line options to silently ignore 24bpp rather than fail

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