drm/i915: Trust programmed MCR in read_subslice_reg

Instead of re-calculating the MCR selector in read_subslice_reg do the
rwm on its existing value and restore it when done.

This consolidates MCR programming to one place for cnl+, and avoids
re-calculating its default value on older platforms during hangcheck.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717180624.20354-3-tvrtko.ursulin@linux.intel.com