[Intel-gfx] Modesetting fixes for pre-ILK, 915GM especially

[Chris Wilson]

I revised the diagnostic patch (assert_fb_bound_for_plane) so that the
function comment actually matches what the function does and ran it on a
couple of machines whilst bug hunting. It scored 2 direct hits. Not only
does intel_get_load_detect_pipe() not attach a framebuffer when enabling
an unused pipe, but the intel_crtc_mode_set() enables the plane before
setting up the memory for the plane to read from.

The latter is quite a scary bug. Can anyone remember if there was a
reason for it to be so?
-Chris