drm/i915: Introduce some local intel_dp variables

The drrs code dereferences mode->vrefresh via some really long chain
of structures/pointers. Couldn't get coccinelle to see through all
that so let's add some local variables to help it.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-3-ville.syrjala@linux.intel.com