mdss: mdp: increase size of mdp image data length

MDP image structure's len member is currently 32-bit.
This field is incorrectly cast to unsigned long before
passing to ion APIs like ion_map_iommu that expect 64-bit
wide data. This can cause unexpected data overwrites.
Avoid such incorrect casts and declare the member
in mdp image structure to have 64-bit width.
Additionally, use appropriate format specifiers in print
statements for the new field type.

Change-Id: I5b60230d25db23f355372284a81ef7505b3e8488
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>