drm_hwcomposer: avoid using signed errno on uint32 type

DrmGenericImporter::ConvertHalFormatToDrm() should not return negative values.

• Use DRM_FORMAT_INVALID instead of -EINVAL
• Check DrmGenericImporter::ConvertHalFormatToDrm() result value in DrmGenericImporter::ImportBuffer()

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