drm/radeon: warn users when hw_i2c is enabled (v2)

The hw i2c engines are disabled by default as the
current implementation is still experimental.  Print
a warning when users enable it so that it's obvious
when the option is enabled.

v2: check for non-0 rather than 1

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org