drm/amdgpu: Fix num_doorbell calculation issue

When paging queue is enabled, it use the second page of doorbell.
The AMDGPU_DOORBELL64_MAX_ASSIGNMENT definition assumes all the
kernel doorbells are in the first page. So with paging queue enabled,
the total kernel doorbell range should be original num_doorbell plus
one page (0x400 in dword), not *2.

Signed-off-by: Oak Zeng <ozeng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>