drm/i915/tgl: Add DC3CO required register and bits

Adding following definition to i915_reg.h
1. DC_STATE_EN register DC3CO bit fields and masks.
   DC3CO enable bit will be used by driver to make DC3CO
   ready for DMC f/w and status bit will be used as DC3CO
   entry status.
2. Transcoder EXITLINE register and its bit fields and mask.
   Transcoder EXITLINE enable bit represents PSR2 idle frame
   reset should be applied at exit line and exitlines mask
   represent required number of scanlines at which DC3CO
   exit happens.

   B.Specs:49196

v1: Use of REG_BIT and using extra space for EXITLINE_ macro
    definition. [Animesh]
v2: Grouping EXITLINE reg bits with EXITLINE(trans) define,
    no functional change. [Ville]

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191007094607.2111-1-anshuman.gupta@intel.com