drm/i915/mst: change naming from fake encoders to MST stream encoders

drm/i915/mst: change naming from fake encoders to MST stream encoders

The fake encoders pretty much match individual MST streams. The encoders
remain as fake as ever, but change the naming to MST stream
encoders. Rename all the encoder hooks and related functions called from
them to mst_stream_* to clarify what type of encoders the hooks are
called on.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/37d9572e884ca2fdd84ea6722b4b7cefde04eed9.1731409802.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>