Fix an incorrect NULL check reported by kbot in the MSM DP driver

smatch warnings:
drivers/gpu/drm/msm/dp/dp_hpd.c:37 dp_hpd_connect()
error: we previously assumed 'hpd_priv->dp_cb' could be null
(see line 37)
Reported-by: Nkernel test robot <lkp@intel.com>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NAbhinav Kumar <abhinavk@codeaurora.org>
Reviewed-by: NStephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/1614971839-2686-2-git-send-email-abhinavk@codeaurora.orgSigned-off-by: NRob Clark <robdclark@chromium.org>