drm/i915/gen9: Fix clearing of the BIOS power well request register

Atm, in the power well sync_hw hook we are clearing all BIOS request
bits, not just the one corresponding to the given power well. This could
turn off an unrelated power well inadvertently if it didn't have a
request bit set in the driver request register.

This didn't cause a problem so far, since we enabled all power wells
explicitly before clearing the BIOS request register. A follow-up
patchset will add power wells that won't get enabled this way, so fix up
the inconsistency.

Note that this patch only makes the clearing of the BIOS req register
more logical. Power wells without a reference would still get disabled
by the end of power domain initialization, that is fixed by the next
patch.

v2:
- Clarify in the commit log that this patch doesn't address the case of
  power wells without a reference. (Ander)

Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487345986-26511-4-git-send-email-imre.deak@intel.com