drm/dsi-staging: reset drm/dsi data before reconfiguring

Memset dsi and drm data structures before populating them
with configuration data to avoid any possibility of using
residue data from previous configurations.

Change-Id: Ia601a540d62445f499b73284d54ec53424aa347d
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>