[Intel-gfx] first attempt at error recovery

[Eric Anholt]

Here's the patch I worked on at today's hack session to attempt error recovery
on top of the error detection I've pushed to drm-intel-next.  It's not
working yet, but maybe some review or others toying with it will help.  My
dmesg from running the hyperspace rss screensaver (which locks up in a couple
seconds):

[ 2059.437548] [drm] Initialized i915 1.6.0 20080730 on minor 0
[ 2063.922100] [drm:i915_report_and_clear_errors] *ERROR* Encountered page table error.
[ 2063.922103] [drm] Can't be cleared without device reset, disabling.
[ 2063.922105] [drm] PGTBL_ER: invalid instruction/state cache PTE
[ 2063.922112] [drm:i915_reset_render_and_ring] *ERROR* Reset the render engine and ringbuffer

no progress occurs after that, though.  Also pushed to error-recovery of my
kernel tree.