drm/msm/sde: Skip unneeded register reads when getting write line count

More often than not, get_vsync_info() is used to only get the write line
count, while the other values it returns are left unused. This is not
optimal since it is done on every display commit. We can eliminate the
superfluous register reads by adding a parameter specifying if only the
write line count is requested.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>