drm/msm/adreno: Add fenced regwrite support

drm/msm/adreno: Add fenced regwrite support

There are some special registers which are accessible even when GX power
domain is collapsed during an IFPC sleep. Accessing these registers
wakes up GPU from power collapse and allow programming these registers
without additional handshake with GMU. This patch adds support for this
special register write sequence.

Signed-off-by: Akhil P Oommen <akhilpo@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/673368/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>