drm/msm: Avoid NULL dereference in msm_disp_state_print_regs()

If the allocation in msm_disp_state_dump_regs() failed then
`block->state` can be NULL. The msm_disp_state_print_regs() function
_does_ have code to try to handle it with:

  if (*reg)
    dump_addr = *reg;

...but since "dump_addr" is initialized to NULL the above is actually
a noop. The code then goes on to dereference `dump_addr`.

Make the function print "Registers not stored" when it sees a NULL to
solve this. Since we're touching the code, fix
msm_disp_state_print_regs() not to pointlessly take a double-pointer
and properly mark the pointer as `const`.

Fixes: 98659487 ("drm/msm: add support to take dpu snapshot")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/619657/
Link: https://lore.kernel.org/r/20241014093605.1.Ia1217cecec9ef09eb3c6d125360cc6c8574b0e73@changeidSigned-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>

drivers/gpu/drm/msm/disp/msm_disp_snapshot_util.c

@@ -48,20 +48,21 @@ static void msm_disp_state_dump_regs(u32 **reg, u32 aligned_len, void __iomem *b
 	}
 }
 
-static void msm_disp_state_print_regs(u32 **reg, u32 len, void __iomem *base_addr,
-		struct drm_printer *p)
+static void msm_disp_state_print_regs(const u32 *dump_addr, u32 len,
+		void __iomem *base_addr, struct drm_printer *p)
 {
 	int i;
-	u32 *dump_addr = NULL;
 	void __iomem *addr;
 	u32 num_rows;
 
+	if (!dump_addr) {
+		drm_printf(p, "Registers not stored\n");
+		return;
+	}
+
 	addr = base_addr;
 	num_rows = len / REG_DUMP_ALIGN;
 
-	if (*reg)
-		dump_addr = *reg;
-
 	for (i = 0; i < num_rows; i++) {
 		drm_printf(p, "0x%lx : %08x %08x %08x %08x\n",
 				(unsigned long)(addr - base_addr),
@@ -89,7 +90,7 @@ void msm_disp_state_print(struct msm_disp_state *state, struct drm_printer *p)
 
 	list_for_each_entry_safe(block, tmp, &state->blocks, node) {
 		drm_printf(p, "====================%s================\n", block->name);
-		msm_disp_state_print_regs(&block->state, block->size, block->base_addr, p);
+		msm_disp_state_print_regs(block->state, block->size, block->base_addr, p);
 	}
 
 	drm_printf(p, "===================dpu drm state================\n");