drm/msm/adreno: get CP_RPTR from register instead of shadow memory

As described in the downstream/kgsl driver:
Sometimes the RPTR shadow memory is unreliable causing timeouts
in adreno_idle().  Read it directly from the register instead.

Change-Id: Ic0dedbadf67029a7f8f872daa16d2286efe46267
Signed-off-by: Craig Stout <cstout@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Git-commit: 7d0c5ee9f07779ab29ac291748060a0a6f1d66db
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>