drm/i915/dsc: Add a power domain for VDSC on eDP/MIPI DSI

On Icelake, a separate power well PG2 is created for
VDSC engine used for eDP/MIPI DSI. This patch adds a new
display power domain for Power well 2.

v3:
* Call it POWER_DOMAIN_TRANSCODER_EDP_VDSC (Ville)
* Move it around TRANSCODER power domain defs (Ville)

v2:
* Fix the power well mismatch CI error (Ville)
* Rename as VDSC_PIPE_A (Imre)
* Fix a whitespace (Anusha)
* Fix Comments (Imre)

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128202628.20238-7-manasi.d.navare@intel.com