intel: Add dri_bufmgr_check_aperture support for bufmgr_gem.

This relies on a new kernel ioctl to get the available aperture size.

In order to provide reasonable performance from dri_bufmgr_check_aperture, we
now require that once a buffer has been used as the target of a relocation,
it gets no further relocations added to it.  This cuts the cost of
check_aperture from 10% to 1% in the 3D driver with no code changes, but
slightly complicates our plans for the 2D driver.