drm/amdgpu: set DRIVER_ATOMIC flag early

The atomic debugfs stuff gets created in drm_dev_alloc()
but this gets called before we've enumerated all of our
IPs, so move the DRIVER_ATOMIC flag setting to fix that.

Since DRIVER_ATOMIC is a driver flag it's currently global
to the driver so setting it affects all GPUs driven by the
driver.  Unfortunately, not all GPUs support atomic.  Warn
the user if that is the case.

This is the same as our current behavior, but at least the
atomic debugfs stuff gets created now.

Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>