drivers: gpu: Mark function as static in radeon_gem.c

Mark function radeon_gem_set_domain() as static in
drm/radeon/radeon_gem.c because it is not used outside this file.

This eliminates the following warning in drm/radeon/radeon_gem.c:
drivers/gpu/drm/radeon/radeon_gem.c:89:5: warning: no previous prototype for ‘radeon_gem_set_domain’ [-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>