Revert "drivers/gpu/drm/i915/intel_overlay.c needs seq_file.h"

This reverts commit e167976e,
Since this was already fixed in commit
3bd3c932 some days before this
commit cause seq_file.h to be included twice.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>