drm: Use C8 instead of RGB332 when determining the format from depth/bpp

Support for real RGB332 is a rarity, most hardware only really support
C8. So use C8 instead of RGB332 when determining the format based on
depth/bpp.

This fixes 8bpp fbcon on i915, since i915 will only accept C8 and not
RGB332.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59572


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Tested-by:  <mlsemon35@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>