msm: mdss: dsi: remove unused memory mapped resource

There are more than one instances where the DSI registers
are memory mapped in the driver whereas only one instance is
being used. Remove the reference to the unused resource.

Change-Id: Ia213e94b43bf58446eead47b18cadeb36d6795a7
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>