drm/i915: Silence compiler warning for hsw_power_well_enable()

Not all compilers are able to determine that pg is guarded by wait_fuses
and so may think that pg is used uninitialized.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: b2891eb2

 ("drm/i915/hsw+: Add has_fuses power well attribute")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171002100416.25865-1-chris@chris-wilson.co.uk
Reviewed-by: Imre Deak <imre.deak@intel.com>