nvc0: avoid using magic numbers for the uniform_bo offsets

nvc0: avoid using magic numbers for the uniform_bo offsets

Instead make use of constants to improve readability.

The first 32 bytes of the driver constant buffer are unknown... This
doesn't seem to be used in the codegen part, but if the texBindBase
offset is shifted from 0x20 to 0x00, this breaks the universe for
really weird reasons. This sounds like to be related to textures.

Anyway, name this NVC0_CB_AUX_UNK_INFO and add a todo should be
enough for now.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>