drm: Ensure proper synchronization when adding/removing i2c adapter

Surround the calls to add/remove the i2c adapter
with a mutex to ensure proper synchronization.

This fixes a race condition in which consecutive
calls to remove an i2c adapter may cause a double
delete in the i2c driver, leading to an invalid
memory access and subsequent crash.

CRs-Fixed: 2511678
Change-Id: I562b029020325b07c977c73f30c4e6af533c7a77
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>