drm/msm: use BUG_ON macro for debugging.

As the if statement only checks for the value of the offset_name
variable, it can be replaced by the more conscise BUG_ON macro for error
reporting.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Rob Clark <robdclark@chromium.org>