drm/msm/sde: read MDSS registers for current configuration

During SDE hardware initialization, read MDP registers
to check the current MDSS configuration. This information
will be used to update software structures when
continuous splash feature is enabled.

Change-Id: Ieb9e80e67a163a3197c882b624a05b1409383aaf
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>