sm501: Misc clean ups

- Rename a variable
- Move variable declarations out of loop to the beginning in draw_hwc_line

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: 187c9e4e09d9bc2967b2454b36bb088ceef0b8bc.1492787889.git.balaton@eik.bme.hu
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>