drm/i915: Add VRR_CTL_LINE_COUNT field to VRR_CTL register def

VRR_CTL register only had a GENMASK but no field prep
define for TRANS_VRR_CTL_LINE_COUNT field so add that

Cc: Aditya Swarup <aditya.swarup@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201202182727.26158-1-manasi.d.navare@intel.com