drm/amdgpu/si: fix crash on headless asics

Missing check for crtcs present.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=193341
https://bugs.freedesktop.org/show_bug.cgi?id=99387

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>