drm/i915: Only apply a rmw mmio update if the value changes

If we try to clear, or even set, a bit in the register that doesn't
change the register state; skip the write. There's a slight danger in
that the register acts as a latch-on-write, but I do not think we use a
rmw cycle with any such latch registers.

Suggested-by: Daniele Ceraolo Spurio <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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