drm/i915: Enable/disable DRRS

Calling enable/disable DRRS when enable/disable DDI are called.
These functions are responsible for setup of drrs data (in enable) and
reset of drrs (in disable).
has_drrs is true when downclock_mode is found and SEAMLESS_DRRS is set in
the VBT. A check has been added for has_drrs in these functions, to make
sure the functions go through only if DRRS will work on the platform with
the attached panel.

V2: [By Ram]: WARN_ON is used when intel_edp_drrs_enable() is called more than
once [Rodrigo]

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>