drm/msm: remove unnecessary NULL check

This NULL check was required when it was added, but we shuffled the code
around and now it's not.  The inconsistent NULL checking triggers a
Smatch warning:

    drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c:847 mdp5_init() warn:
    variable dereferenced before check 'mdp5_kms' (see line 782)

Fixes: 1f50db2f ("drm/msm/mdp5: move resource allocation to the _probe function")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/562559/
Link: https://lore.kernel.org/r/ZSj+6/J6YsoSpLak@kadam


Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>