drm/i915: Allocate a drm_atomic_state for the legacy modeset code

For the atomic conversion, the mode set paths need to be changed to rely
on an atomic state instead of using the staged config. By using an
atomic state for the legacy code, we will be able to convert the code
base in small chunks.

v2: Squash patch that adds stat argument to intel_set_mode(). (Ander)
    Make every caller of intel_set_mode() allocate state. (Daniel)
    Call drm_atomic_state_clear() in set config's error path. (Daniel)

v3: Copy staged config to atomic state in force restore path. (Ander)

v4: Don't update ->new_config for disabled pipes in __intel_set_mode(),
    since it is expected to be NULL in that case. (Ander)

v5: Don't change return type of intel_modeset_pipe_config(). (Chandra)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
[danvet: Remove spurious ret local variable due to changes in v5.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>