drm/i915: Preserve the state of power wells not explicitly enabled

Atm, power wells that BIOS has enabled, but which we don't explicitly
enable during power domain initialization would get disabled as we clear
the BIOS request bit in the given power well sync_hw hook. To prevent
this copy over any set request bits in the BIOS request register to the
driver request register and clear the BIOS request bit only afterwards.

This doesn't make a difference now, since we enable all power wells
during power domain initialization. A follow-up patchset will add power
wells for which this isn't true, so fix up the inconsistency.

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/1487345986-26511-5-git-send-email-imre.deak@intel.com