Add HW context support to i915 driver

Starting with Gen6 (Sandy Bridge), Mesa 9.2+ relies on hardware context switching in the i915 driver.

The patch: https://people.freebsd.org/~dumbbell/graphics/drm-i915-hw-context.2.patch

The branch is on GitHub: https://github.com/dumbbell/freebsd/tree/kms-i915-hw-context

We received two reports with the previous patch. Both users reported instability or display artifacts. The new revision of the patch includes bugfixes from Linux 3.8.13.

We're insterested in test reports (success or failure) on any GPU (even non-Intel).

1. Install Mesa 10.2 from the "experimental" branch of the xorg-dev repository. You need to rebuild at least the following ports:
   • graphics/libdrm
   • graphics/libglapi
   • graphics/libGL
   • graphics/dri
2. Check if Mesa complains about missing hw context:

# glxinfo
name of display: :0.0
Gen6+ requires Kernel 3.6 or later.
Assertion failed: (ctx->Version > 0), function handle_first_current, file ../../src/mesa/main/context.c, line 1498.
Abort (core dumped)

1. Clone the Git branch or apply the patch to your local source tree
2. Build and install kernel
3. Reboot
4. Re-run glxinfo

# glxinfo
name of display: :0
display: :0  screen: 0
direct rendering: Yes
...
OpenGL renderer string: Mesa DRI Intel(R) 965GM 
OpenGL version string: 2.1 Mesa 10.2.4
...

1. Play with your usual GL applications.

Please report any success or failure by following the instructions on the Graphics portal.