intel: remove the drm_mm* symbol workarounds

Added with commit 57b4c4c3(Move the renaming of mm.c symbols to
symbol duplication/collision with ones that are available elsewhere.

As the public/private symbols of libdrm are properly annotated neither
one of the symbols will end up in the global name-space, thus should no
longer be required.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>