drm/bridge: ti-sn65dsi86: Use regmap_bulk_write API

The multi-register u16 write operation can use regmap_bulk_write()
instead of two separate regmap_write() calls.

It's uncertain if this has any effect on the actual updates of the
underlying registers, but this at least gives the hardware the
opportunity and saves us one transation on the bus.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025170925.3096444-2-bjorn.andersson@linaro.org