drm/ast: Properly initialize P2A base before using it in ast_init_3rdtx()

drm/ast: Properly initialize P2A base before using it in ast_init_3rdtx()

If the P2A has been used to target other SOC registers before that
call, we're going to hit the wrong place so make sure we set the
base address up properly before using it.

(P2A stands for PCIe to AHB bridge and is the bride that allows
accessing the AST's internal AHB bus using a relocatable 64k
window in the second half of the PCIe MMIO BAR)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>