drm/i915: Remove duplicate include of intel_region_lmem.h

Fix the following checkinclude.pl warning:
drivers/gpu/drm/i915/gt/intel_region_lmem.c
8	#include "intel_region_lmem.h"
     12	#include "intel_region_lmem.h"

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210615113522.6867-1-wanjiabing@vivo.com