drm/msm: clarify downstream bus scaling

A few spots in the driver have support for downstream android
CONFIG_MSM_BUS_SCALING.  This is mainly to simplify backporting the
driver for various devices which do not have sufficient upstream
kernel support.  But the intentionally dead code seems to cause
some confusion.  Rename the #define to make this more clear.

Signed-off-by: Rob Clark <robdclark@gmail.com>