drm/stm: remove unneeded -Iinclude/drm compiler flag

With the include directives under include/drm/ fixed, this flag is
no longer needed.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1495081793-9707-13-git-send-email-yamada.masahiro@socionext.com

drivers/gpu/drm/stm/Makefile

-ccflags-y := -Iinclude/drm
-
 stm-drm-y := \
 	drv.o \
 	ltdc.o