msm: mdss: mdp: Fix the intf count

In MDP_DISP_INTF_SEL register, it uses different value to represent the
display type. It cannot assume enabled intf is always 1.

CRs-Fixed: 1078202
Change-Id: I73e36f9bb369f3200b99f66fb5231a0afe389913
Signed-off-by: Camus Wong <camusw@codeaurora.org>