drm/i915: Hold runtime PM during plane commit

drm/i915: Hold runtime PM during plane commit

During plane operations, we read/write some registers that only operate
properly if we're not runtime suspended.  At the moment we're not
holding the runtime PM reference across the whole plane operation, so
there's a potential for problems.

This issue was already partially addressed by commit

        commit 8e0c5934135f178baa6fe604c81d313771d35fbc
        Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
        Date:   Fri Aug 15 15:59:32 2014 -0300

            drm/i915: fix plane/cursor handling when runtime suspended

which took care of holding the runtime PM reference during the pin and
fence operations for plane updates.  However there are still a few
actual plane registers that we also need to hold the runtime PM
reference for.  Recent refactoring patches in preparation for atomic
have rearranged the code and made it increasingly likely that the
hardware will have time to suspend between the pin/fence operation and
the actual register writes. Examples of such registers are the stuff
touched by ivb_get_colorkey.

The solution here grabs the runtime PM reference around the 'commit'
operation for planes, which should cover all the relevant register
reads/writes.

Note that this has only been exposed with

commit e9675f53cd4c2b5b65a30b6d476e80804551938e
Author: Matt Roper <matthew.d.roper@intel.com>
Date:   Mon Dec 1 15:40:14 2014 -0800

    drm/i915: Consolidate plane 'prepare' functions (v2)

so doesn't need to be ported to 3.19.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87180
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Testcase: igt/pm-rpm/legacy-planes
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Augment commit message with information Paulo supplied.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>