sm501: Get rid of base address in draw_hwc_line

Do not use the base address to access data in local memory. This is in
preparation to allow chip connected via PCI where base address depends
on where the BAR is mapped so it will be unknown.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Message-id: 79dab21bc6ec4d563aabf265c3bab40e2e95aae8.1492787889.git.balaton@eik.bme.hu
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>