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>

drivers/gpu/drm/radeon/atom.h

@@ -154,6 +154,13 @@ bool atom_parse_data_header(struct atom_context *ctx, int index, uint16_t *size,
 bool atom_parse_cmd_header(struct atom_context *ctx, int index,
 			   uint8_t *frev, uint8_t *crev);
 int atom_allocate_fb_scratch(struct atom_context *ctx);
+
+struct i2c_msg;
+struct i2c_adapter;
+int radeon_atom_hw_i2c_xfer(struct i2c_adapter *i2c_adap,
+			    struct i2c_msg *msgs, int num);
+u32 radeon_atom_hw_i2c_func(struct i2c_adapter *adap);
+
 #include "atom-types.h"
 #include "atombios.h"
 #include "ObjectID.h"

drivers/gpu/drm/radeon/radeon_i2c.c

@@ -34,10 +34,6 @@
 #include "radeon.h"
 #include "atom.h"
 
-extern int radeon_atom_hw_i2c_xfer(struct i2c_adapter *i2c_adap,
-				   struct i2c_msg *msgs, int num);
-extern u32 radeon_atom_hw_i2c_func(struct i2c_adapter *adap);
-
 bool radeon_ddc_probe(struct radeon_connector *radeon_connector, bool use_aux)
 {
 	u8 out = 0x0;