drm/amd/display: Add doc entry for program_3dlut_size

Fixes the warning:

Function parameter or struct member 'program_3dlut_size' not described in
'mpc_funcs'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/dri-devel/20240715090445.7e9387ec@canb.auug.org.au/


Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>