drm/vc4: plane: Optimize the LBM allocation size

The current code is using the maximum of the source line size and the
destination line size to compute the size of the LBM to allocate.

While this is simpler, it starts to be an issue with modes such as 4k with
a quite long that will consume all the available memory, so we no longer
have that luxury.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b9e091883a4f7395c5b6a4f7c6070225934293db.1599120059.git-series.maxime@cerno.tech