drm/amd/display: Fix kernel docs for new struct members

[WHAT & HOW]
Add kernel-doc for new struct members for lut3d_caps and mpc_funcs.

This fixes the follow warnings from "make htmldocs".

./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:1106: warning: Function
parameter or struct member 'mcm' not described in 'mpc_funcs'
./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:1106: warning: Function
parameter or struct member 'rmcm' not described in 'mpc_funcs'
./drivers/gpu/drm/amd/display/dc/dc.h:249: warning: Function parameter
or struct member 'mcm_3d_lut_caps' not described in 'mpc_color_caps'
./drivers/gpu/drm/amd/display/dc/dc.h:249: warning: Function parameter
or struct member 'rmcm_3d_lut_caps' not described in 'mpc_color_caps'
./drivers/gpu/drm/amd/display/dc/dc.h:249: warning: Function parameter
or struct member 'preblend' not described in 'mpc_color_caps'

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>