drm/amd/display: Don't pass invalid fw_bss_data pointer into DMUB srv

[Why]
If bss_data_size is 0 then we shouldn't be passing down fw_bss_data into
the DMUB service since the region isn't really "valid."

[How]
Pass NULL instead if the size is 0.

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>