drm/amdgpu: Initialize crtc, pageflip irq funcs (v2)

For virtual display feature, initialize dce_virtual_crtc_irq_funcs,
dce_virtual_pageflip_irq_funcs. As it has no dce engine, so the
pageflip interrupt won't be generated, and the vsync interrupt will
be generated by smu's periodic timer or software timer which will
be implemented later.

v2: agd: rebase on upstream

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