drm/amdgpu: drop drm_firmware_drivers_only()

There are a number of systems and cloud providers out there
that have nomodeset hardcoded in their kernel parameters
to block nouveau for the nvidia driver.  This prevents the
amdgpu driver from loading. Unfortunately the end user cannot
easily change this.  The preferred way to block modules from
loading is to use modprobe.blacklist=<driver>.  That is what
providers should be using to block specific drivers.

Drop the check to allow the driver to load even when nomodeset
is specified on the kernel command line.

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