msm: kgsl: Get CP_RPTR from register instead of shadow memory
Sometimes the RPTR shadow memory is unreliable causing timeouts
in adreno_idle().  Read it directly from the register instead.

Change-Id: Icae7521edd9723610c41f320c6a5c99f57bec7f5
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>