disp: msm: dsi: remove scratch register logic for cont-splash

Continuous splash enabled displays are identified by reading
the MDP ctl registers. DSI cont-splash init settings are
called based on this. Additionally, DSI reads the DSI-CTL
scratch register set by bootloader  to detect cont-splash.
This change removes the redundant mechanism in DSI to
detect cont-splash.

Change-Id: Ic58be1e62eda239fcea5e82d9d356905dc552a73
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>