drm/vc4: Move the buffer offset out of the vc4_plane_state

The offset fields in vc4_plane_state are described as being
the offset for each buffer in the bo, however it is used to
store the complete DMA address that is then written into the
register.

The DMA address including the fb ofset  can be retrieved
using drm_fb_dma_get_gem_addr, and the offset adjustment due to
clipping is local to vc4_plane_mode_set.
Drop the offset field from the state, and compute the complete
DMA address in vc4_plane_mode_set.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>