drm/vc4: Fix pitch setup for T-format scanout.

drm/vc4: Fix pitch setup for T-format scanout.

The documentation said to use src_w here, and I didn't consider that
we actually needed to be using pitch somewhere in our setup.  Fixes
scanout on my DSI panel when X11 does initial setup with 1920x1080
HDMI and 800x480 DSI both at 0,0 of the same framebuffer.

v2: Add some comments requested by Boris

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 8653cf8cd8ab ("drm/vc4: Add T-format scanout support.")
Link: https://patchwork.freedesktop.org/patch/msgid/20170927193209.11870-1-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>