drm/amd/display: Fix dc/dcn20/dcn20_optc.c kdoc

Fix all kdoc warnings in dc/dcn20/dcn20_optc.c:

display/dc/dcn20/dcn20_optc.c:41: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Enable CRTC
display/dc/dcn20/dcn20_optc.c:76: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 *For the below, I'm not sure how your GSL parameters are stored in your
  env,
display/dc/dcn20/dcn20_optc.c:85: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * There are (MAX_OPTC+1)/2 gsl groups available for use.

Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>