drm/i915: avoid non-atomic sysrq execution

The sysrq functions are executed in hardirq context, so we shouldn't be
calling sleeping functions from them, like mutex_locks or memory
allocations.

Fix up the i915 sysrq handler to avoid this.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>