drivers: gpu: Mark functions as static in radeon_device.c

Mark functions radeon_doorbell_init() and radeon_doorbell_fini() as
static in drm/radeon/radeon_device.c because they are not used outside
this file.

This eliminates the following warning in drm/radeon/radeon_device.c:
drivers/gpu/drm/radeon/radeon_device.c:252:5: warning: no previous prototype for ‘radeon_doorbell_init’ [-Wmissing-prototypes]
drivers/gpu/drm/radeon/radeon_device.c:281:6: warning: no previous prototype for ‘radeon_doorbell_fini’ [-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>