drm/i915/gen9_bc: Work around DMC bug zeroing power well requests

drm/i915/gen9_bc: Work around DMC bug zeroing power well requests

A DMC bug on GEN9 big core machines fails to restore the driver's
request bits for the PW1 and MISC_IO power wells after a DC5/6
entry->exit sequence. As a consequence the driver's subsequent check for
the enabled status of these power wells will fail, as the check
considers the power wells being enabled only if both the status and
request bits are set. To work around this borrow the request bits from
BIOS's own request register in which DMC forces on the request bits when
exiting from DC5/6.

This fixes a problem reported by Ramalingam, where HDCP init failed,
since PW1 reported itself as being disabled, while in reality it was
enabled.

Reported-by: Ramalingam C <ramalingam.c@intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109145822.15446-1-imre.deak@intel.com