drm/vc4: Drop planes that are completely off-screen

It is permitted for a plane to be configured such that none
of it is on-screen via either negative dest rectangle X,Y
offset, or just an offset that is greater than the crtc
dimensions.

These planes were resized via drm_atomic_helper_check_plane_state
such that the source rectangle had a zero width or height, but
they still created a dlist entry even though they contributed
no pixels. In the case of vc6_plane_mode_set, that it could result
in negative values being written into registers, which caused
incorrect behaviour.

Drop planes that result in a source width or height of 0 pixels
to avoid the incorrect rendering.

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