drivers: gpu: Move prototype declarations to header file radeon_mode.h from radeon_atombios.c and radeon_combios.c

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

This eliminates the following warnings in drm/radeon/radeon_encoders.c:
drivers/gpu/drm/radeon/radeon_encoders.c:86:1: warning: no previous prototype for ‘radeon_get_encoder_enum’ [-Wmissing-prototypes]
drivers/gpu/drm/radeon/radeon_encoders.c:162:1: warning: no previous prototype for ‘radeon_link_encoder_connector’ [-Wmissing-prototypes]
Signed-off-by: NRashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>