Fixes the warning:

Function parameter or struct member 'bb_from_dmub' not described in 'amdgpu_display_manager'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>