drm/i915: make intel_uncore_rmw() write unconditionally

Two small changes in intel_uncore_rmw will allow to use it more broadly:
- write register unconditionally, for use with latch registers,
- return old value of the register, IRQ cleanup and similar.

If we really want to keep write-only-if-changed feature maybe other
helper will be more suitable for it, intel_uncore_rmw name suggests
unconditional write.

Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006163200.2803722-4-andrzej.hajda@intel.com