drm/amd/display: Add NULL pointer checks in dc_stream cursor attribute functions

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Add NULL pointer checks in dc_stream cursor attribute functions

The function dc_stream_set_cursor_attributes() currently dereferences
the `stream` pointer and nested members `stream->ctx->dc->current_state`
without checking for NULL.

All callers of these functions, such as in
`dcn30_apply_idle_power_optimizations()` and
`amdgpu_dm_plane_handle_cursor_update()`, already perform NULL checks
before calling these functions.

Fixes below:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:336 dc_stream_program_cursor_attributes()
error: we previously assumed 'stream' could be null (see line 334)

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c
327 bool dc_stream_program_cursor_attributes(
328 struct dc_stream_state *stream,
329 const struct dc_cursor_attributes *attributes)
330 {
331 struct dc *dc;
332 bool reset_idle_optimizations = false;
333
334 dc = stream ? stream->ctx->dc : NULL;
^^^^^^
The old code assumed stream could be NULL.

335
--> 336 if (dc_stream_set_cursor_attributes(stream, attributes)) {
^^^^^^
The refactor added an unchecked dereference.

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c
313 bool dc_stream_set_cursor_attributes(
314 struct dc_stream_state *stream,
315 const struct dc_cursor_attributes *attributes)
316 {
317 bool result = false;
318
319 if (dc_stream_check_cursor_attributes(stream, stream->ctx->dc->current_state, attributes)) {
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Here.
This function used to check for if stream as NULL and return false at
the start. Probably we should add that back.