drm/bridge: samsung-dsim: use while loop in samsung_dsim_transfer_start

drm/bridge: samsung-dsim: use while loop in samsung_dsim_transfer_start

Turn the open-coded goto-again construct into a while loop, to make
samsung_dsim_transfer_start() a bit shorter and easier to read.

Hold the spinlock when looping back around and avoid the duplicated
list_empty() check.

Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Link: https://lore.kernel.org/r/20250527-samsung-dsim-v1-1-5be520d84fbb@pengutronix.de
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>