drm/msm/mdp5: Fix iteration on INTF config array

The current iteration in get_dsi_id_from_intf() is wrong:
instead of iterating until hw_cfg->intf.count, we need to iterate
until MDP5_INTF_NUM_MAX here.

Let's take the example of msm8x16:

 hw_cfg->intf.count = 1
 intfs[0] = INTF_Disabled
 intfs[1] = INTF_DSI

If we stop iterating once i reaches hw_cfg->intf.count (== 1),
we will miss the test for intfs[1].

Actually, this hw_cfg->intf.count entry is quite confusing and is not
(or *should not be*) used anywhere else; let's remove it.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>