drm/i915: Add HW context support

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

This an MFC of r271705.

Approved by:	re (glebius)
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>,
Relnotes:	yes