drm/i915: Get PM ref before accessing HW register

Seeing these errors when GT is likely in suspend state-
"RPM wakelock ref not held during HW access"

Ensure GT is awake before trying to access HW registers. Avoid
reading the register if that is not the case.

Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Fixes: 41e5c17e ("drm/i915/guc/slpc: Sysfs hooks for SLPC")
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210907232704.12982-1-vinay.belgaumkar@intel.com