sm501: Use values from the pitch register for 2D operations

Before, crt_h_total was used for src_width and dst_width. This is a
property of the current display setting and not relevant for the 2D
operation that also can be done off-screen. The pitch register's purpose
is to describe line pitch relevant of the 2D operation.

Signed-off-by: Sebastian Bauer <mail@sebastianbauer.info>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>