drm/i915/vlv: Turn off power gate for BIOS-less system.

During system boot up, by default, the power gate for render, media and
display well still power gated.  Normally, BIOS will turn off the power
gate.  In the BIOS-less system, the driver need to turn off the power
gate very early during driver load.

v2: Move this to intel_uncore_sanitize to allow it to get call during
resume path. (Daniel)
v3: Remove redundant write 0 to DPIO_CTL, and use DPIO_RESET instead of
just 0x1 (Ville)
    Add turn of power gate for display 2d/render well/media well.
v4: Remove toggle cmnreset in intel_uncore_sanitize.  Cmnreset should
toggle after CRI clock source has been selected.  Jesse DPIO reset patch
which toggle the cmnreset in intel_modeset_init_hw() should handle it.
(Ville)

Signed-off-by: Chon Ming Lee <chon.ming.lee@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>