drm/bridge/synopsys: dsi: allow sending longer LP commands

drm/bridge/synopsys: dsi: allow sending longer LP commands

Current code does not properly computes the max length of LP
commands that can be send during H or V sync, and rely on static
values.
Limiting the max LP length to 4 byte during the V-sync is overly
conservative.

Relax the limit and allows longer LP commands (16 bytes) to be
sent during V-sync.

Signed-off-by: Antonio Borneo <antonio.borneo@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701143131.841-1-yannick.fertre@st.com