drm/i915/display: Disable DRRS when needed in fastsets

Changes in the configuration could cause PSR to be compatible and
enabled so driver must also be able to disable DRRS when doing
fastsets.

v2: Fixed name of DRRS compute function (Anshuman)

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/209
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/173
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/209


Cc: Srinivas K <srinivasx.k@intel.com>
Cc: Hariom Pandey <hariom.pandey@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200825171331.17971-2-jose.souza@intel.com