msm: kgsl: Use ERR_PTR to return errors from kgsl_create_context()

Normally kgsl_create_context() returns a pointer to a new context.
In case of errors it just returns NULL pointer, and that doesn't allow
to propagate the right error code back to the user via
kgsl_ioctl_drawctxt_create()
This fix modifies kgsl_create_context to use ERR_PTR macro for passing
error codes to its caller.

Change-Id: I447c1765828912b0994bcae67a73864e62eef9b6
Signed-off-by: Vladimir Razgulin <vrazguli@codeaurora.org>