drm/amd/amdgpu: For virtual display, enable multi crtcs. (v3)

Enable multi crtcs for virtual display, user can set the number of crtcs
by amdgpu module parameter  virtual_display.

v2: make timers per crtc
v3: agd: simplify implementation

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