msm: kgsl: always check kgsl_active_count_get() result

Getting an active count can fail if the hardware doesn't
actually wake up. Make active count get functions __mustcheck
to remind people of this and fix existing calls to
handle the error.

Change-Id: I5a0202c824ac7e436b9061da6d8f638e44e8b7f6
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>