drm/radeon: add a i2c bus mutex

The i2c and aux buses use the same pads so add
a mutex to protect access to the pads.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>

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

diff --cc drivers/gpu/drm/radeon/atombios_i2c.c
Simple merge

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

diff --cc drivers/gpu/drm/radeon/radeon_mode.h
Simple merge