libdrm: fix the Android 64bit build

With 64bit bionic mmap now handles 64bit offset, thus we no longer
need the __mmap2 trick.

Fix from Chih-Wei Huang, over at the google forums.

Cc: Chih-Wei Huang <cwhuang@android-x86.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>