drm/amdgpu: Correct the irq numbers for virtual crtc

The irq number should be decided by num_crtc, and the num_crtc could change
by parameter.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>