swrast: swrastFillInModes doesn't do 8-bit modes, so don't try

Support for 8-bit modes was removed in commits 0398a26 and bda208a4.
However, I didn't notice code in dri_init_screen that explicitly tries
to create this modes.  This is structurally different from other drivers
(that only create modes that match the display color depth).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56555
Cc: Vinson Lee <vlee@freedesktop.org>
1 file changed