drm/i915/tgl: Implement TGL DisplayPort training sequence

On TGL some registers moved from DDI to transcoder and the
DisplayPort training sequence has a separate BSpec page.

I started adding 'ifs' to the original intel_ddi_pre_enable_dp() but
it was becoming really hard to follow, so a new and cleaner function
for TGL was added with comments of all steps. It's similar to ICL,
but different enough to deserve a new function.

The rest of DisplayPort enable and the whole disable sequences
remained the same.

v2: FEC and DSC should be enabled on sink side before start link
training(Maarten reported and Manasi confirmed the DSC part)

v3: Add call to enable FEC on step 7.l(Manasi)

BSpec: 49190
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823082055.5992-16-lucas.demarchi@intel.com