This combines the two cmdstream bits "is_3d" and "is_not_cubemap" into a
single 2-bit texture target selection, noticing it's the same as the
2-bit selection in Midgard and Bifrost texturing ops. Accordingly, we
share this definition and add the missing entry for 1D/buffer textures.

This requires a nontrivial (but functionally similar) refactor of all
parts of the driver to use the new definitions appropriately.
Theoretically, this should add support for buffer textures, but that's
obviously not tested and probably wouldn't work.

While doing so, we notice the sRGB enable bit, which we document and
decode as well here so we don't forget about it.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>