parisc: Flush gatt writes and adjust gatt mask in parisc_agp_mask_memory()

[ Upstream commit d703797380c540bbeac03f104ebcfc364eaf47cc ]

Flush caches after changing gatt entries and calculate entry according
to SBA requirements.

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>