drm/msm: match wait_for_completion_timeout return type

return type of wait_for_completion_timeout is unsigned long not int, this
patch assigns the return value of wait_for_completion_timeout to an
appropriately typed and named variable.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>