drm/amdgpu: adjust drm_firmware_drivers_only() handling

Move to probe so we can check the PCI device type and
only apply the drm_firmware_drivers_only() check for
PCI DISPLAY classes.  Also add a module parameter to
override the nomodeset kernel parameter as a workaround
for platforms that have this hardcoded on their kernel
command lines.

Reviewed-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>