msm: mdss: Fix multiple bridge chip usecase

To allow multiple instances of ADV7533 bridge chips, we need to set
different instance ids for each bridge chip instead of hardcoding.
Also, intialise switch dev only if the panel is pluggable.

Change-Id: I1f4ae6a84c869da9ae551ed54dc3b4f22293c94a
Signed-off-by: Siddharth Zaveri <szaveri@codeaurora.org>