drm: drop drm_pcie_get_speed_cap_mask and drm_pcie_get_max_link_width

These functions duplicated functionality which was ultimately added
to the pci core.

All users of these functions have been ported to using the newly
exposed pci functionality.  These functions are no longer used,
so drop them.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>