display: hwc_buffer_allocator fixes

Fixes an issue where the HWC init randomly blocks on the
allocator init by doing a lazy initialization of the allocator
object. The allocator may not have fully initialized at HWC init.
Also, increases the allocator connection threadpool to 4.

Change-Id: I38454ee25aaf2a211125e55297300eac754fb5cf
CRs-Fixed: 2201202
4 files changed