[3.0] avcodec: video: fix swfmt detection when using a hw_device_ctx

(cherry picked from commit 906ffbaa) (edited) edited:

• 3.0 uses PixelFormat
• the code to handle b_hardware_only was already merged in 3.0

Signed-off-by: Steve Lhomme robux4@ycbcr.xyz

For hardware-only decoders, the swfmt is always AV_PIX_FMT_NONE.

Fixes #28220 (closed)