Documentation/gpu: add remaining DOC: comments to Intel display documentation

Not all of the overview DOC: comments in the display driver are
incorporated into the documentation. Add the missing one

Documentation/gpu: add remaining DOC: comments to Intel display documentation

Not all of the overview DOC: comments in the display driver are
incorporated into the documentation. Add the missing ones, including
some function documentation.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patch.msgid.link/589c46cf9a46763f4fbb7e1756656e6d71ba1431.1778235406.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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