drm/dp: add a hw mutex around the transfer functions. (v2)

This should avoid races between connector probing and HPD
irqs in the future, currently mode_config.mutex blocks this
possibility.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/drm_dp_helper.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_dp.c
Simple merge

diff --cc drivers/gpu/drm/radeon/atombios_dp.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_i2c.c
Simple merge

diff --cc drivers/gpu/drm/tegra/dpaux.c
Simple merge

diff --cc include/drm/drm_dp_helper.h
Simple merge