drm/amdgpu: ih doorbell size of range changed for nbio v7.4

[Why]
nbio v7.4 size of ih doorbell range is 64 bit. This requires 2 DWords per register.

[How]
Change ih doorbell size from 2 to 4. This means two Dwords per ring.
Current configuration uses two ih rings.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>