drm/radeon: remove unneeded header include path

A header include path without $(srctree)/ is suspicious because it does
not work with O= builds.

You can build drivers/gpu/drm/radeon/ without this include path.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/Makefile

@@ -3,8 +3,6 @@
 # Makefile for the drm device driver.  This driver provides support for the
 # Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.
 
-ccflags-y := -Idrivers/gpu/drm/amd/include
-
 hostprogs := mkregtable
 clean-files := rn50_reg_safe.h r100_reg_safe.h r200_reg_safe.h rv515_reg_safe.h r300_reg_safe.h r420_reg_safe.h rs600_reg_safe.h r600_reg_safe.h evergreen_reg_safe.h cayman_reg_safe.h