The path for the "mod_info_packet.h" header file is
incomplete, so add its location to the header search path
in the amdgpu Makefile.

See on ARCH=alpha (275 times in one build).

In file included from ../drivers/gpu/drm/amd/amdgpu/amdgpu.h:90,
                 from ../drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c:43:
../drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.h:62:10: fatal error: mod_info_packet.h: No such file or directory
   62 | #include "mod_info_packet.h"
      |          ^~~~~~~~~~~~~~~~~~~
compilation terminated.

Fixes: 5b49da02 ("drm/amd/display: Enable Freesync over PCon")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>