drm: omapdrm: Remove duplicate error messages when mapping memory

The devm_ioremap_resource() call can handle being given a NULL resource,
and prints an error message when mapping fails. Switch the remaining
devm_ioremap() calls to devm_ioremap_resource() and remove all
extraneous resource NULL checks and error messages printed manually by
the driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>