drm/i915: Add support for DRRS to switch RR

This patch computes and stored 2nd M/N/TU for switching to different
refresh rate dynamically. PIPECONF_EDP_RR_MODE_SWITCH bit helps toggle
between alternate refresh rates programmed in 2nd M/N/TU registers.

v2: Daniel's review comments
Computing M2/N2 in compute_config and storing it in crtc_config

v3: Modified reference to edp_downclock and edp_downclock_avail based on the
changes made to move them from dev_private to intel_panel.

v4: Modified references to is_drrs_supported based on the changes made to
rename it to drrs_support.

v5: Jani's review comments
Removed superfluous return statements. Changed support for Gen 7 and above.
Corrected indentation. Re-structured the code which finds crtc and connector
from encoder. Changed some logs to be less verbose.

v6: Modifying i915_drrs to include only intel connector as intel_dp can be
derived from intel connector when required.

v7: As per internal review comments, acquiring mutex just before accessing
drrs RR. As per Chris's review comments, added documentation about the use
of locking in the function.

v8: Incorporated Jani's review comments.
Removed reference to edp_downclock.

v9: Jani's review comments. Modified comment in set_drrs. Changed index to
type edp_drrs_refresh_rate_type. Check if PSR is enabled before setting
registers fo DRRS.

Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>