drm/bridge: fix stack usage warning on old gcc

Some older versions of gcc badly optimize code that passes
an inline function argument into another function by reference,
causing huge stack usage:

drivers/gpu/drm/bridge/tc358768.c: In function 'tc358768_bridge_pre_enable':
drivers/gpu/drm/bridge/tc358768.c:840:1: error: the frame size of 2256 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

Use a temporary variable as a workaround and add a comment pointing
to the gcc bug.

Fixes: ff1ca639

 ("drm/bridge: Add tc358768 driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428215408.4111675-1-arnd@arndb.de