drm/amd/display: Fix up kdoc formatting in display_mode_vba.c

drm/amd/display: Fix up kdoc formatting in display_mode_vba.c

Fixes the following W=1 kernel build warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dml/display_mode_vba.c:936: warning: Cannot understand  * *************************************************************************

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