drm/amd/display: Check bss_data_size before going down legacy DMUB load path

[Why]
New unified firmware binary with only inst const still passes down
fw_bss_data != NULL and params->bss_data_size == 0 from DM.

This leads it into the legacy path causing firmware state allocation to
be too small.

[How]
Check bss_data_size as well.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>