drm/amd/display: Use NULL pointer instead of plain integer

Assigning 0L to a pointer variable caused the following warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dsc/rc_calc_fpu.c:71:40:
warning: Using plain integer as NULL pointer

In order to remove this warning, this commit assigns a NULL pointer to
the pointer variable that caused this issue.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>