drm/radeon/kms: take the pm mutex when using hw i2c

we need a constant sclk for i2c prescale.  Also,
get the current sclk when calculating prescale rather
than using the default sclk.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>