drm/radeon/kms: Fix I2C mask definitions

Commit 9b9fe724 accidentally used RADEON_GPIO_EN_* where
RADEON_GPIO_MASK_* was intended. This caused improper initialization
of I2C buses, mostly visible when setting i2c_algo_bit.bit_test=1.
Using the right constants fixes the problem.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Jerome Glisse <j.glisse@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>