drm/i915: Unify power domain handling.

Right now there's separate power domain handling for update_pipe and
modesets. Unify this and only grab POWER_DOMAIN_MODESET once.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455108583-29227-3-git-send-email-maarten.lankhorst@linux.intel.com


Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>