msm: kgsl: Remove duplicate code for checking timestamps

Event based as well as interruptible based timestamp checks
use the same code, they differ on if the mutex is held or not.
Separate the common code out to avoid confusion.

Change-Id: Ie2ee01d7a07ff2ba7755ab8d327796345f9ca9ea
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>
Signed-off-by: Rajeev Kulkarni <krajeev@codeaurora.org>