usbpd: Add mutex lock to manipulate svid_handler list

svid_handler is registered by different client driver (e.g. Linux
DP driver). Client driver may unregister in some error case which
may race against accessing svid_handler list. Hence add mutext lock
to guard svid_handler list manipulation.

CRs-Fixed: 2102703
Change-Id: I6653a31e3fa1f27e0cdd7a32f752df57f8b6c718
Signed-off-by: Mayank Rana <mrana@codeaurora.org>