gcx: source setup optimization.

Share the same setup routines for blitting and
scaling. Use old-style registers for source 0
in blit setup due to a hardware problem.

Change-Id: Ic9ed5c95d37833521f2eda21d818f8a8696bcd4c
Signed-off-by: Alexei Shlychkov <x0177296@ti.com>