drm/i915: Kill check_power_well() calls

pps_{lock,unlock}() call intel_display_power_{get,put}() outside
pps_mutes to avoid deadlocks with the power_domain mutex. In theory
during aux transfers we should usually have the relevant power domain
references already held by some higher level code, so this should not
result in much overhead (exception being userspace i2c-dev access).
However thanks to the check_power_well() calls in
intel_display_power_{get/put}() we end up doing a few Punit reads for
each aux transfer. Obviously doing this for each byte transferred via
i2c-over-aux is not a good idea.

I can't think of a good way to keep check_power_well() while eliminating
the overhead, so let's just remove check_power_well() entirely.

Fixes a driver init time regression introduced by:
 commit 773538e8
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Thu Sep 4 14:54:56 2014 +0300

    drm/i915: Reset power sequencer pipe tracking when disp2d is off

Credit goes to Jani for figuring this out.

v2: Add the regression note in the commit message.

Cc: stable@vger.kernel.org (v3.18+)
Cc: Egbert Eich <eich@suse.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86201


Tested-by: Wendy Wang <wendy.wang@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>