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>
diff --git a/Android.bp b/Android.bp
index 5681cb7..85be0b5 100644
--- a/Android.bp
+++ b/Android.bp
@@ -110,6 +110,15 @@
     include_dirs: ["external/minigbm/cros_gralloc"],
 }
 
+// Used by hwcomposer.drm_imagination
+filegroup {
+    name: "drm_hwcomposer_platformimagination",
+    srcs: [
+        "platform/platformdrmgeneric.cpp",
+        "platform/platformimagination.cpp",
+    ],
+}
+
 // Used by hwcomposer.drm_hikey and hwcomposer.drm_hikey960
 filegroup {
     name: "drm_hwcomposer_platformhisi",