drm/i915: Add HW context support

This feature is required by Mesa 9.2+. Without this, a GL application
crashes with the following message:

1. 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)

Now, Mesa 10.2.4 and 10.3-rc3 works fine:

1. 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 ...

The code was imported from Linux 3.8.13.

Reviewed by: kib@
Tested by: kwm@, danfe@, Henry Hu,

		Lundberg, Johannes <johannes@brilliantservice.co.jp>,
		Johannes Dieterich <dieterich.joh@gmail.com>,
		Lutz Bichler <lutz.bichler@gmail.com>,

MFC after: 3 days
Relnotes: yes