i965/surface_formats: Don't advertise 8 or 16-bit RGB formats

i965/surface_formats: Don't advertise 8 or 16-bit RGB formats

We have implicitly been not advertising these formats since we had them
turned off in the format capabilities table.  We are about to update that
table and this prevents a change in behavior.  The only change in behavior
created by this patch is that we no longer advertise support for
R16G16B16_FLOAT which means that it's now renderable which seems like a
bonus.  Maybe someday we'll want to change things to start supporting
16-bit RGB formats natively but, at the moment, there's no need.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>