drm/gma500: 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/1493009447-31524-19-git-send-email-yamada.masahiro@socionext.com

drivers/gpu/drm/gma500/Makefile

 #
 #	KMS driver for the GMA500
 #
-ccflags-y += -I$(srctree)/include/drm
 
 gma500_gfx-y += \
 	  accel_2d.o \