drm/i915: wrap GEN6_PMIMR changes

Just like we're doing with the other IMR changes.

One of the functional changes is that not every caller was doing the
POSTING_READ.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>