drm/radeon/atom: Move 'radeon_atom_hw_i2c_*()'s prototypes into shared header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/atombios_i2c.c:100:5: warning: no previous prototype for ‘radeon_atom_hw_i2c_xfer’ [-Wmissing-prototypes]
 100 | int radeon_atom_hw_i2c_xfer(struct i2c_adapter *i2c_adap,
 | ^~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/atombios_i2c.c:150:5: warning: no previous prototype for ‘radeon_atom_hw_i2c_func’ [-Wmissing-prototypes]
 150 | u32 radeon_atom_hw_i2c_func(struct i2c_adapter *adap)
 | ^~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>