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>

drivers/gpu/drm/amd/display/dc/dml/dsc/rc_calc_fpu.c

@@ -68,7 +68,7 @@ static void get_qp_set(qp_set qps, enum colour_mode cm, enum bits_per_comp bpc,
 	int sel = table_hash(mode, bpc, max_min);
 	int table_size = 0;
 	int index;
-	const struct qp_entry *table = 0L;
+	const struct qp_entry *table = NULL;
 
 	// alias enum
 	enum { min = DAL_MM_MIN, max = DAL_MM_MAX };