drm: only build ati_pcigart if PCI enabled

Prevent ati_pcigart.c being built unless PCI is enabled. The exported
functions in this file are only used by drivers which depend on PCI
(namely r128 and radeon), and it tries to use PCI specific functions
(pci_unmap_page, pci_map_page, and pci_dma_mapping_error) that cause
compiler errors when PCI is disabled.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>