drm/sde: add panel count to support multiple bridge chips

When multiple bridge chips are connected to same interface, the
drm mode in the mode_set has combined timing parameters. For
each individual bridge chip, those timing parameters need to
be divided by panel count.

CRs-Fixed: 1085590
Change-Id: I9af0fa99ab6bcf9e09f4f7b372d53e6f1638e6d0
Signed-off-by: Jin Li <jinl@codeaurora.org>