drm/i915: kill i915_mem.c

Some decent history digging indicates that this was to be used for the
GLX_MESA_allocate_memory extension but never actually implemented for
any released i915 userspace code.

So just rip it out.

v2: Fixup the Makefile.

Acked-by: Dave Airlie <airlied@gmail.com>
Cc: Keith Whitwell <keithw@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>