backports: fix intel i915 drm driver

backports: fix intel i915 drm driver

The intel i915 drm driver uses the declarations from intel-gtt.h to
interact with drivers/char/agp/intel-gtt.c which is not
backported by backports project. To prevent some strange problems when
using i915, use the headers from the kernel and do not use our own. This
causes some compile problems which where runtime problems before and
bumps the minimal kernel version to 3.7.

This i915 still crash on my device with kernel 3.7.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>