drm/i915: stop conflating HAS_DISPLAY() and disabled display

Stop setting ->pipe_mask to zero when display is disabled, allowing us
to have different code paths for not actually having display hardware,
and having display hardware disabled. This lets us develop those two
avenues independently.

There are no functional changes for when there is no display. However,
all uses of for_each_pipe() and for_each_pipe_masked() will start
running for the disabled display case. Put one of the more significant
ones behind checks for INTEL_DISPLAY_ENABLED(), otherwise the cases
should not be hit with disabled display, or they seem benign. Fingers
crossed.

All in all, this might not be the ideal solution. In fact we may have
had something along the lines of this in the past, but we ended up
conflating the two cases. Possibly even by recommendation by yours
truly; I did not dare dig up that part of the history. But the perfect
is the enemy of the good, this is a straightforward change, and lets us
get actual work done in both fronts without interfering with each other.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190916092901.31440-1-jani.nikula@intel.com