gralloc4-vts: Allow YCRCB_420_SP to be unsupported in Lock_YCRCB_420_SP

Some devices may not support the legacy YCRCB_420_SP format. To allow
the test can pass through such devices, the patch adds the flexibility
when UNSUPPORTED is returned from buffer allocation.

Bug: 150461327
Bug: 152510209
Test: VtsHalGraphicsMapperV4_0Target

Change-Id: I393fc3c4a7d2421f07eeff88915041c92e8cdf05