drm/amdgpu: bypass bios dependent operations

Since bios reading does not work currently so just bypass all operations
related to bios

v2: hardcode the vram info for APP_APU case (hawking)
v3: correct the vram_width with channel number * channel size (lijo)

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>