drm/i915: Introduce a gmbus power domain

Currently the gmbus code uses intel_aux_display_runtime_get/put in an
effort to make sure the hardware is powered up sufficiently for gmbus.
That function only takes the runtime PM reference which on VLV/CHV/BXT
is not enough. We need the disp2d/pipe-a well on VLV/CHV and power well
2 on BXT. So add a new power domnain for gmbus and kill off the now
unused intel_aux_display_runtime_get/put. And change
intel_hdmi_set_edid() to use the gmbus power domain too since that's all
we need there.

Also toss in a BUILD_BUG_ON() to catch problems if we run out of
bits for power domains. We're already really close to the limit...

[Patrik: Add gmbus string to debugfs output]

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
[Cherry-picked from drm-intel-next-queued f0ab43e6

 (Imre)]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448643329-18675-3-git-send-email-imre.deak@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>