mdss: mdp: parse memory handle for cont splash buffer

Continuous splash screen buffer is reserved in
target dtsi file for MSM8994 target. FB driver should
parse this memory handle using "linux,continuous-region"
to get the splash screen buffer address and size. This
avoids usage of "qcom,memory-reserve" entry which forces
user to provide duplicate information.

Change-Id: Ice6cddf6c71a2de9adf15a763434a310e07a3da6
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>