drm/panel: use mipi_dsi_dcs_write_buffer where possible

A few of the new panels create a local macro wrapping around
mipi_dsi_dcs_write. At the same time, they don't really care about the
command/payload split.

mipi_dsi_dcs_write does a kmalloc/memcpy/kfree for payload > 7 bytes.
Avoid that all together by using the _buffer function.

Aside:
panel-xinpeng-xpp055c272.c calls its wrapper "generic" although it
should be "dcs". But that for another day/patch.

Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200505160329.2976059-2-emil.l.velikov@gmail.com