display-legacy: clarify COPYBIT == HWC_BLIT

Change-Id: I4e531764a782a583312d3734f4fa0d3a4604b667
display-legacy: Use TARGET_QCOM_DISPLAY_VARIANT flag for usage.

Change-Id: Ia348226eeb7bda8368edac4a7086ed4413a6597d
Merge "display-legacy: clarify COPYBIT == HWC_BLIT" into jellybean