drm/amdgpu: make amdgpu_display_framebuffer_init() static

It's not used outside of amdgpu_display.c.

Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>