drm/i915: remove i915_disable_vga_mem declaration

It was supposed to have been killed on the same commit that killed the
function, e1264ebe9ff48e1b3e1dd11805eec9f5b143ab7c, but I guess the
intel_drv.h reorganization accidentally brought it back.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>