drm_hwcomposer: Add Imagination platform support

drm_hwcomposer: Add Imagination platform support

External Android.bp file should be created in order to build this module:
```
cc_library_shared {
    name: "hwcomposer.drm_imagination",
    defaults: ["hwcomposer.drm_defaults"],
    srcs: [":drm_hwcomposer_platformimagination"],
    whole_static_libs: ["drm_hwcomposer"],
    shared_libs: ["libion"],
    include_dirs: [
        "path/to/imgtec/include/files",
    ],
}
```
libion is needed to make ion.h header visible `linux/ion.h`.

Signed-off-by: Roman Stratiienko <roman.stratiienko@globallogic.com>