drop MOD_INVALID, MOD_LINEAR definitions

MOD_INVALID came with libdrm 2.4.83 and MOD_LINEAR came with libdrm
2.4.82. libweston unconditionally depends on libdrm >= 2.4.95, so the
fallback is not necessary.

Since linux-dmabuf.h itself has no use for these and also forgets to
include drm_fourcc.h, .c files including drm_fourcc.h after this header
would trigger compiler warnings.

linux-dmabuf.c does need these, so add the proper include.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/backend-drm/kms.c

@@ -42,10 +42,6 @@
 #include "pixel-formats.h"
 #include "presentation-time-server-protocol.h"
 
-#ifndef DRM_FORMAT_MOD_LINEAR
-#define DRM_FORMAT_MOD_LINEAR 0
-#endif
-
 struct drm_property_enum_info plane_type_enums[] = {
 	[WDRM_PLANE_TYPE_PRIMARY] = {
 		.name = "Primary",