media: stm32: dma2d: remove unused fb_buf

Drop the unused struct v4l2_framebuffer fb_buf in struct dma2d_ctx.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
CC: Hugues Fruchet <hugues.fruchet@foss.st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>