drm/msm/dpu: Initialize crtc_state to NULL in dpu_plane_virtual_atomic_check()

After a recent change in clang to expose uninitialized warnings from
const variables and pointers [1], there is a warning around crtc_state
in dpu_plane_virtual_atomic_check():

  drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1145:6: error: variable 'crtc_state' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
   1145 |         if (plane_state->crtc)
        |             ^~~~~~~~~~~~~~~~~
  drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1149:58: note: uninitialized use occurs here
   1149 |         ret = dpu_plane_atomic_check_nosspp(plane, plane_state, crtc_state);
        |                                                                 ^~~~~~~~~~
  drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1145:2: note: remove the 'if' if its condition is always true
   1145 |         if (plane_state->crtc)
        |         ^~~~~~~~~~~~~~~~~~~~~~
   1146 |                 crtc_state = drm_atomic_get_new_crtc_state(state,
  drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1139:35: note: initialize the variable 'crtc_state' to silence this warning
   1139 |         struct drm_crtc_state *crtc_state;
        |                                          ^
        |                                           = NULL

Initialize crtc_state to NULL like other places in the driver do, so
that it is consistently initialized.

Cc: stable@vger.kernel.org
Closes: https://github.com/ClangBuiltLinux/linux/issues/2106
Fixes: 774bcfb7 ("drm/msm/dpu: add support for virtual planes")
Link: https://github.com/llvm/llvm-project/commit/2464313eef01c5b1edf0eccf57a32cdee01472c7

 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>