drm/msm: gpu: Cut down the list of "generic" registers to the ones we use

There are very few register accesses in the common code. Cut down
the list of common registers to just those that are used.  This
saves const space and saves us the effort of maintaining registers
for A3XX and A4XX that don't exist or are unused.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>