drm_hwcomposer: Try to fix build with older Android Pie releases

Try to fix build regressions for P that were introduced with
commit b3d81781 ("drm_hwcomposer: Add GetDisplayCapabilities and
getDisplayIdentificationData") as noted in issue #30
  https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer/issues/30

Change-Id: I4bfeb952cff19d4c3511ead69df1186d3a04cc87
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/include/drmhwctwo.h b/include/drmhwctwo.h
index 76e71b0..89ae2f6 100644
--- a/include/drmhwctwo.h
+++ b/include/drmhwctwo.h
@@ -173,11 +173,13 @@
                                    uint32_t *num_elements, hwc2_layer_t *layers,
                                    int32_t *layer_requests);
     HWC2::Error GetDisplayType(int32_t *type);
+#if PLATFORM_SDK_VERSION > 28
     HWC2::Error GetDisplayIdentificationData(uint8_t *outPort,
                                              uint32_t *outDataSize,
                                              uint8_t *outData);
     HWC2::Error GetDisplayCapabilities(uint32_t *outNumCapabilities,
                                        uint32_t *outCapabilities);
+#endif
     HWC2::Error GetDozeSupport(int32_t *support);
     HWC2::Error GetHdrCapabilities(uint32_t *num_types, int32_t *types,
                                    float *max_luminance,