drm/msm/sde: Cache register values when performing clock control

Remote register I/O amounts to a measurably significant portion of CPU
time due to how frequently this function is used. Cache the value of
each register on-demand and use this value in future invocations to
mitigate the expensive I/O.

Co-authored-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>
Change-Id: I62f5b19f8c4ee62521432a60a76350181c84ec86