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.

Signed-off-by: Craig Stout <cstout@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>