drm/msm/sde: sanitize debugfs inputs when reading mdp memory

Sanitize debugfs inputs to only allow access to mdp memory block
specified in dtsi file. This change will allow only one single block
to be read at the time and will avoid accessing memory outside of valid
decode space which can trigger AHB error bus response.

Change-Id: I91b91ac2b31eac4facb0c402c92ed8fcfc01d91e
Signed-off-by: Amine Najahi <anajahi@codeaurora.org>