gpu: drm: amd: display: fix kernel-doc warnings

Fix kernel-doc warnings discovered in AMD gpu display driver.
Fixes these warnings:
./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:110: warning:
Function parameter or member 'overlap_only'
not described in 'mpcc_blnd_cfg'.

./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:110: warning:
Function parameter or member 'bottom_gain_mode'
not described in 'mpcc_blnd_cfg'.

./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:110: warning:
Function parameter or member 'background_color_bpc'
not described in 'mpcc_blnd_cfg'.

./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:110:
warning: Function parameter or member 'top_gain'
not described in 'mpcc_blnd_cfg'.

./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:110:
warning: Function parameter or member 'bottom_inside_gain'
not described in 'mpcc_blnd_cfg'.

./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:110:
warning: Function parameter or member 'bottom_outside_gain'
not described in 'mpcc_blnd_cfg'.

Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Swarup Laxman Kotiaklapudi <swarupkotikalapudi@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>