drm/amdgpu: add debug flag to place fw bo on vram for frontdoor loading

Use debug_mask=0x8 param to help isolating data path issues
on new systems in early phase.

v2: rename the flag for explicitness (lijo)

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>