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

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

This eliminates the following warning in drm/radeon/radeon_kms.c:
drivers/gpu/drm/radeon/radeon_kms.c:194:5: warning: no previous prototype for ‘radeon_info_ioctl’ [-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>