drm/amd/display: Use fb_base/fb_offset if available for translation

[Why]
In the case where we do have these passed in then we should be using
these instead of the registers - those are only programmed when booting
the adapter as primary.

[How]
Check if fb_base/fb_offset are non-zero. The value of 0 is invalid
for fb_base but is valid for fb_offset on dGPU.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dmub/src/dmub_dcn20.c

@@ -60,6 +60,12 @@ static void dmub_dcn20_get_fb_base_offset(struct dmub_srv *dmub,
 {
 	uint32_t tmp;
 
+	if (dmub->fb_base || dmub->fb_offset) {
+		*fb_base = dmub->fb_base;
+		*fb_offset = dmub->fb_offset;
+		return;
+	}
+
 	REG_GET(DCN_VM_FB_LOCATION_BASE, FB_BASE, &tmp);
 	*fb_base = (uint64_t)tmp << 24;