The offset adjustment depends on the framebuffer modified, so let's
just move this operation in the DRM_FORMAT_MOD_LINEAR case inside
vc4_plane_mode_set().

This we'll be able to fix offset calculation for
DRM_FORMAT_MOD_BROADCOM_VC4_T_TILED and DRM_FORMAT_MOD_BROADCOM_SANDXXX.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180803092231.26446-4-boris.brezillon@bootlin.com