drm/dsi-staging: do not update the stored values in parser

While retrieving the data from the DSI parser, do not update
the stored values. APIs to retrieve the stored data can be
called multiple times. Updating the stored values may result
in wrong data to be returned in subsequent calls.

CRs-Fixed: 2223812
Change-Id: I189639a1451a067e70452e3f06a85f488a40e7e0
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>