gpu: host1x: Rework CPU syncpoint increment

This patch merges host1x_syncpt_cpu_incr to host1x_syncpt_incr() as
they are in practise doing the same thing. host1x_syncpt_incr() is
also modified to return error codes. User space interface is modified
accordingly to pass return values.

Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Acked-By: Terje Bergstrom <tbergstrom@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>