msm: kgsl: Don't open kgsl device in erroneous case

If kgsl_get_process_private() returns NULL, it goes to
err_stop with result = -ENOMEM. If open_count is 0,
adreno_stop() overwrites the result variable with 0.
This leads to kgsl_open() to be successful while it
shouldn't.

Change-Id: Ic12d92a099280e4d958fa192fa5128211120d361
Signed-off-by: Prabhat Awasthi <pawasthi@codeaurora.org>