msm: kgsl: Avoid deadlock when deleting process sysfs nodes

The process mutex is held while removing the sysfs nodes, which
waits for any access to finish before completion. If another thread
does a read at the same time, it will wait for the process mutex
which is already being held by the first thread, causing a deadlock.

Get a refcount to the process private to make sure that it is valid
while the sysfs nodes exist. This removes the need to hold the
process mutex while reading the nodes.

CRs-Fixed: 796805
Change-Id: I01fd721eb6cbd7443abcc5eaf13a86844efd61b6
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>