drm/i915: Remove use_mmio_flip modparm, v2.

This has been unused since commit afa8ce5b
("drm/i915: Nuke legacy flip queueing code").

Changes since v1:
- Rebase on top of all the changes to modparams.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
\o/-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171004094416.31306-1-maarten.lankhorst@linux.intel.com