drm/i915: remove user GTT mappings early during runtime suspend

Currently user space can access GEM buffers mapped to GTT through
existing mappings concurrently while the platform specific suspend
handlers are running. Since these handlers may change the HW state in a
way that would break such accesses, remove the mappings before calling
the handlers. Spotted by Ville.

Also Chris pointed out that the lists that i915_gem_release_all_mmaps()
walks through need dev->struct_mutex, so take this lock. There is a
potential deadlock against a concurrent RPM resume, resolve this by
aborting and rescheduling the suspend (Daniel).

v2:
- take struct_mutex around i915_gem_release_all_mmaps() (Chris, Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Robert Beckett <robert.beckett@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>