drm/amdgpu/gfx: make amdgpu_gfx_me_queue_to_bit() static

It's not used outside of amdgpu_gfx.c.

Reviewed-by: Sunil Khatri <sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>