msm: mdss: Fix potential NULL pointer dereference in mdss smmu driver

FB driver will call this function even if there is no MDP.
mdss_mdp_get_mdata will return NULL in that scenario and needs to be
checked.

Change-Id: Icebed1bc9b4da214fec2ca924590403d1656fa5d
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>