msm: mdss: add support for rotator in mdss_mdp_mixer_get

Currently ctl->mfd is not populated for rotator sessions. If
mdss_mdp_mixer_get is called by rotator session then it will always
fail the NULL check for ctl->mfd. This will flood the logs for every
rotator commit and can lead to un-predictable outcome. Fix this by
properly handling rotator case in mdss_mdp_mixer_get.

Change-Id: Ib1896149302bd60b99028ae26df2393fab30dd6e
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>