disp: msm: sde: consider num DS for mixer width

Helper function is returning the width of single mixer. During
atomic crtc check, the number of destination scaler is not
considered before width check and can fail possible higher
resolutions. The mixerwidth should be taken as a multiple of
num_ds_enabled.

Change-Id: Id9e5e0ccf0cebb54d2a242e039d8dc3676b3729f
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>