compat-drivers: address usage of intel_enable_gtt() for i915

compat-drivers: address usage of intel_enable_gtt() for i915

Kernels older than 3.6 don't have intel_enable_gtt() exported
and most Linux distributions have CONFIG_AGP_INTEL as built-in
(y) and as such we don't bother to make it available as
modular here. The change to export intel_enable_gtt() was
made through commit 8ecd1a66 but previous to this the driver
never called intel_enable_gtt() so we can safely assume we
can do the same for older kernels.

There should be a way to export an intel_enable_gtt() for
kernels with CONFIG_AGP_INTEL as built-in but that itself
is a project.

commit 8ecd1a6615f0d9de6759aafe229bc1cc4ee99c7b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jun 7 15:56:03 2012 +0200

    drm/i915: call intel_enable_gtt

    When drm/i915 is in control of the gtt, we need to call
    the enable function at all the relevant places ourselves.

    Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
This now goes with i915 run time tested against 3.2.0-3-amd64
and code extrated from linux-next next-20120926.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>