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

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>