media: aspeed: set hsync and vsync polarities to normal before starting mode detection

Sometimes it detects a weird resolution such as 1024x287 when the
actual resolution is 1024x768. To resolve such an issue, this
commit adds clearing for hsync and vsync polarity register bits
at the beginning of the first mode detection. This is recommended
in the datasheet.

Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>