drm/msm/gpu: Add a comment in a6xx_gmu_init()

If you don't realize is_a650_family() also encompasses a660 family,
you'd think that the debug buffer is double allocated.  Add a comment
to make this more clear.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20211124214151.1427022-11-robdclark@gmail.com


Signed-off-by: Rob Clark <robdclark@chromium.org>