drm/amdgpu: check gfx pipe availability before toggling its interrupts

GUI_IDLE interrupts controlled by CP_INT_CNTL_RING0
are only applicable to me0 pipe0.

For ASICs that have gfx pipe removed, don't toggle
those bits.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>