sm501: Fix hardware cursor

Rework HWC handling to simplify it and fix cursor not updating on
screen as needed. Previously cursor was not updated because checking
for changes in a line overrode the update flag set for the cursor but
fixing this is not enough because the cursor should also be updated if
its shape or location changes. Introduce hwc_invalidate() function to
handle that similar to other display controller models.

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: 6970a5e9868b7246656c1d02038dc5d5fa369507.1492787889.git.balaton@eik.bme.hu
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
