drm/i915: Track pfit enable state separately from size

Detangle the additional state of whether or not the hw has the pfit
enabled from whether it has zero size. This allows us to cleanly
distinguish in the code when we expect the pfit to be enabled (for
Haswell pc8), and when the BIOS is confused and needs sanitizing.

Reported-by: shui yanwei <yangweix.shui@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68251


Tested-by: shui yanwei <yangweix.shui@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>