drm/amd/display: Get fb base and fb offset for DMUB from registers

[Why]
Under some hardware initialization sequences the fb base/fb offset
provided can be zero or hardwareinit can happen too late.

We want to ensure that we always have the correct fb_base/fb_offset
when performing DMCUB hardware initialization so we can do DMCUB
command table offloading during first dc hardware init.

[How]
Read from the DCN registers. VBIOS already filled these in for us.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>