disp: msm: sde: use mdp scratch register to pass drm mode info

Use MDP scratch registers to pass drm mode index information
between the VMs. Expose the APIs which LA VM can use to set
the current mode for all active displays before handoff.
LE VM relies on this information to set the corresponding
display in correct mode during handoff.

Change-Id: I4569dd58e953e588bca816ac718335d3f3f7e29b
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>