drm/i915: Clarify gpu_error.lock locking

i915_capture_error_state can be called from all kinds of contexts, so
needs the full irqsave dance. But the other two places to grab and
release the error state are only called from process context. So
simplify them to the plaine _irq spinlock versions to clarify the
locking semantics.

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>