i915 / PM: Fix hibernate regression caused by suspend/resume splitting

Commit 84b79f8d (drm/i915: Fix crash
while aborting hibernation) attempted to fix a regression introduced
by commit cbda12d7 (drm/i915:
implement new pm ops for i915), but it went too far trying to split
the freeze/suspend and resume/thaw parts of the code.  As a result,
it introduced another regression, which only is visible on some systems.

Fix the problem by merging i915_drm_suspend() with
i915_drm_freeze() and moving some code from i915_resume()
into i915_drm_thaw(), so that intel_opregion_free() and
intel_opregion_init() are also executed in the freeze and thaw code
paths, respectively.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reported-and-tested-by: Pedro Ribeiro <pedrib@gmail.com>
Tested-by: Tino Keitel <tino.keitel@tikei.de>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>