msm: audio: qdsp6v2: Add NULL check for mmap APR handle

In addtion to mmap ref_cnt check, add condition to check for
mmap APR handle NULL before calling APR register.

As part adsp subsystem restart handling, mmap APR handle is set to
NULL in reset event handling. Checking for mmap APR handle will
allow APR registration to succeed post adsp subsystem restart.

Change-Id: Id1945467616c1c0331b0627d14dabdbc24a0acdd
Signed-off-by: Ravishankar Sarawadi <crsaraw@codeaurora.org>