libcamera: raspberrypi: dmaheaps: Add isValid()

Add isValid() method to verify the allocator device has been open
correctly.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/pipeline/raspberrypi/dma_heaps.h

@@ -18,6 +18,7 @@ class DmaHeap
 public:
 	DmaHeap();
 	~DmaHeap();
+	bool isValid() const { return dmaHeapHandle_ > -1; }
 	FileDescriptor alloc(const char *name, std::size_t size);
 
 private: