drm/i915: FIFO space query code refactor

When querying the GTFIFOCTL register to check the FIFO space, the read value
must be masked. The operation is repeated explicitly in several places. This
change refactors the read-and-mask code into a function call.

v2: rebased on top of Mika's forcewake patch set, specifically:
	[PATCH 8/8] drm/i915: Enum forcewake domains and domain identifiers

Change-Id: Id1a9f3785cb20b82d4caa330c37b31e4e384a3ef
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>