drm/amdgpu: improve MSI-X handling (v3)

Check the number of supported vectors and fall back to MSI if
we return or error or 0 MSI-X vectors.

v2: only allocate one vector.  We can't currently use more than
one anyway.

v3: install the irq on vector 0.

Tested-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Shaoyun liu <shaoyun.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>