drivers: gpu: Move prototype declaration to header file radeon_mode.h

Move prototype declaration of functions radeon_add_atom_connector() and
radeon_add_legacy_connector() to header file drm/radeon/radeon_mode.h
because  they are used by more than one file.

This eliminates the following warning in drm/radeon/radeon_connectors.c:
drivers/gpu/drm/radeon/radeon_connectors.c:1588:1: warning: no previous prototype for ‘radeon_add_atom_connector’ [-Wmissing-prototypes]
drivers/gpu/drm/radeon/radeon_connectors.c:2020:1: warning: no previous prototype for ‘radeon_add_legacy_connector’ [-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>