drm/amdgpu: switch to query reserved fb size from vbios (v3)

For Sienna_Cichlid, query fw_reserved_fb_size from vbios directly.
For navi1x, fall back to default 64K TMR size.
For pre-navi, no need to reserve tmr region in top LFB.

v2: fix TMR define (Alex)
v3: partially revert size change

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>