drm/amdgpu: make program_imu_rlc_ram static

This symbol is not used outside of imu_v11_0.c, so marks it
static.

Fixes the following w1 warning:

drivers/gpu/drm/amd/amdgpu/imu_v11_0.c:302:6: warning: no previous
prototype for ‘program_imu_rlc_ram’ [-Wmissing-prototypes].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>