drm/amd/display: Fix S4 hang polling on HW power up done for VBIOS DMCUB

[Why]
VBIOS DMCUB firmware doesn't set the dal_fw bit and we end up hanging
waiting for HW power up done because of it.

[How]
Simplify the path and allow mailbox_rdy to be a functional check when
we detect VBIOS firmware.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>