drm/i915: add header search path to subdir Makefiles

With the subdirectories we lost the ability to build individual files on
the command line, for example:

$ make drivers/gpu/drm/i915/display/intel_display.o

This was due to the top level directory missing from header search
path. Add the header search paths to subdir Makefiles.

Note that none of the other options in the top level i915 Makefile are
taken into account when building individual files. Usually this is not a
concern.
Reported-by: NImre Deak <imre.deak@intel.com>
Reported-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626143618.21800-2-jani.nikula@intel.com

drivers/gpu/drm/i915/display/Makefile

+# For building individual subdir files on the command line
+subdir-ccflags-y += -I$(srctree)/$(src)/..
+
 # Extra header tests
 include $(src)/Makefile.header-test

drivers/gpu/drm/i915/gem/Makefile

-include $(src)/Makefile.header-test # Extra header tests
+# For building individual subdir files on the command line
+subdir-ccflags-y += -I$(srctree)/$(src)/..
+
+# Extra header tests
+include $(src)/Makefile.header-test

drivers/gpu/drm/i915/gt/Makefile

+# For building individual subdir files on the command line
+subdir-ccflags-y += -I$(srctree)/$(src)/..
+
 # Extra header tests
 include $(src)/Makefile.header-test