drm/i915: Restarting the Idleness DRRS in drrs_flush

Corrected the documentation on the intel_edp_drrs_flush and
intel_edp_drrs_invalidate.

And accordingly edp_drrs_flush function is modified to restart the idleness
detection after upclocking.

v2: Update kerneldoc

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> (v1)
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>