drm/amdgpu: drop amdgpu_display_gem_fb_init()

Unused.

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>