drm/i915: Disable GEM when a broken video BIOS takes up the whole aperture.

This is seen on some G41 systems, where the BIOS will consume all but
a few KB of the aperture.  This should be bad for all operating systems, as
it means that the OS can't dynamically manage memory between graphics and
the rest of the system, and OSes that did static memory management
statically add memory in addition to the BIOS allocation anyway.  So, instead
of working around it, just fail out verbosely.

fd.o bug #21574

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>