drm/msm: Remove redundant check for 'submit'

Rectify the below smatch warning:
drivers/gpu/drm/msm/msm_gem_submit.c:963 msm_ioctl_gem_submit() warn:
variable dereferenced before check 'submit'

'submit' is normally error pointer or valid, so remove its NULL
initializer as it's confusing and also remove a redundant check for it's
value.

Signed-off-by: Aashish Sharma <shraash@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>