drm/bridge: ti-sn65dsi86: Combine register accesses in ti_sn_aux_transfer()

These register reads and writes are sometimes directly next to each
other in the register address space. Let's use regmap bulk read/write
APIs to get the data with one transfer instead of multiple i2c
transfers. This helps cut down on the number of transfers in the case of
something like reading an EDID where we read in blocks of 16 bytes at a
time and the last for loop here is sending an i2c transfer for each of
those 16 bytes, one at a time. Ouch!

Changes in v3:
 - Undid changes in v2

Changes in v2:
 - Combined AUX_CMD register write

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102181144.3469197-2-swboyd@chromium.org