drm/i915: fix comment for POWER_DOMAIN_TRANSCODER_VDSC_PW2

The power domain covers VDSC for DSI transcoder on ICL, and it's
pedantically about pipe, not transcoder, on TGL.

Reported-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191219133845.9333-1-jani.nikula@intel.com