drm/i915/tgl: Do modeset to enable and configure DC3CO exitline

DC3CO enabling B.Specs sequence requires to enable end configure
exit scanlines to TRANS_EXITLINE register, programming this register
has to be part of modeset sequence as this can't be change when
transcoder or port is enabled.
When system boots with only eDP panel there may not be real
modeset as BIOS has already programmed the necessary registers,
therefore it needs to force a modeset to enable and configure
DC3CO exitline.

v1: Computing dc3co_exitline crtc state from a DP encoder
    compute config. [Imre]
    Enabling and disabling DC3CO PSR2 transcoder exitline from
    encoder pre_enable and post_disable hooks. [Imre]
    Computing dc3co_exitline instead of has_dc3co_exitline bool. [Imre]
v2: Code refactoring for symmetry and to avoid exported function. [Imre]
    Removing IS_TIGERLAKE check from compute_config, adding PIPE_A
    restriction and clearing dc3co_exitline state if crtc is not active
    or it is not PSR2 capable in dc3co exitline compute_config. [Imre]
    Using GEN >= 12 check in dc3co exitline get_config. [Imre]

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: 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/20191003081738.22101-5-anshuman.gupta@intel.com