lavu/hwcontext_d3d: Cast src pointers calling av_image_copy*().

lavu/hwcontext_d3d: Cast src pointers calling av_image_copy*().

Silences several warnings:
libavutil/hwcontext_d3d11va.c:413:49: warning: passing argument 3 of ‘av_image_copy’ from incompatible pointer type
libavutil/hwcontext_d3d11va.c:425:47: warning: passing argument 3 of ‘av_image_copy’ from incompatible pointer type
libavutil/hwcontext_dxva2.c:351:45: warning: passing argument 3 of ‘av_image_copy’ from incompatible pointer type
libavutil/hwcontext_dxva2.c:382:52: warning: passing argument 3 of ‘av_image_copy_uc_from’ from incompatible pointer type