Move prototype declaration of function radeon_atom_copy_swap() to header
file drm/radeon/radeon_mode.h because it is used by more than one file.

This eliminates the following warnings in drm/radeon/atombios_dp.c:
drivers/gpu/drm/radeon/atombios_dp.c:53:6: warning: no previous prototype for ‘radeon_atom_copy_swap’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>