drm/i915/dsi/xelpd: Disable DC states in Video mode

drm/i915/dsi/xelpd: Disable DC states in Video mode

MIPI DSI transcoder cannot be in video mode to support any of the
display C states.

Bspec: 49195 (For DC*co DSI transcoders cannot be in video mode)
Bspec: 49193 (Hardware does not support DC5 or DC6 with MIPI DSI enabled)
Bspec: 49188 (desc of DSI_DCSTATE_CTL talks about cmd mode PM control

v2: Align to the power domain ordering (Jani)
    Add bspec references (Imre)

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211019151435.20477-4-vandita.kulkarni@intel.com