drm/omap: avoid copy in mgr_fld_read/write

Avoid unnecessary copy in mgr_fld_read/write by taking a pointer to the
reg_resc and using that.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190930103840.18970-3-tomi.valkeinen@ti.com


Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>