drm/omap: reorganize locking in mgr_fld_write

Fix sparse warning:

drivers/gpu/drm/omapdrm/dss/dispc.c:387:9: warning: context imbalance in 'mgr_fld_write' - different lock contexts for basic block

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>