drm/i915: Allow MI_LOAD_REGISTER_REG between whitelisted registers.

Allowing register copies where the source and destination are both
whitelisted should be safe, and is useful.  For example, Mesa uses
this to load the command streamer math registers with data from the
pipeline statistics counters.

v2: Reject writes to OACONTROL (and reads as well :(

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> # v1
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1462521014-13595-1-git-send-email-chris@chris-wilson.co.uk