drm/i915: abstract display suspend/resume operations

drm/i915: abstract display suspend/resume operations

Increase abstraction of display suspend/resume operations by providing
higher level functions, and hiding the details inside
intel_display_power.c.

v2: Make checkpatch happy:
    - braces {} are not necessary for single statement blocks
v3: Also move hsw/bdw PC8 sequences since they are related to
    display PM anyways. (Ville)
v4: Rebase after a long time, plus Move functions to the new
    intel_display_power so we can stop exporting platform specific
    functions as pointed by Jani.
v5: Remove unnecessary braces.
v6 by Jani: make this purely non-functional cleanup, make functions static

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190806122208.16786-2-jani.nikula@intel.com