hwc: For FB target check RGB pipe availability before VG pipe

If RGB pipe is available then it should be allocated before
allocating VG pipe for FB target.

Change-Id: I0c1f0fd08a7779eb077e9e9c1809bdbcaff2227c
hwc: Disable mdp comp if mDownscaleMode is set for external

If mDownscaleMode is on then scaling will be done. Sending a RGB
layer with alpha to VG pipe can cause composition issues.

Change-Id: I215a5d7360fe741124e809be0b66c328c943d7dc
hwc: bring back fb support for primary for mdp3.

mdp3 devices don't support Overlay display commit so currently all mdp303 devices need this.

Change-Id: I98cf0fdae6b9b1100c97ac319ab1c321f55fe507
display: Conditionally restore genlock support

 * Enable genlock support with QCOM_BSP_WITH_GENLOCK.
 * Some blobs need both genlock and metadata.

Change-Id: Idd1df589516534a683e0fa1ef0cfbb7f0e411f67