disp: msm: sde: always enable prog fetch and fix prefill calculations

Enabling and disabling programmable fetch dynamically
across different fps can cause dsi underflow/overflow followed
by underrun. Add changes to always enable prog fetch to handle
such cases. Fix needed_prefill_lines calculation for vfp method
of dfps since linetime is constant in this method and hence
prefill lines need to be based on max_fps. For panels whose
linetime varies with fps, the needed_prefill_lines calculation
remains unchanged.

Change-Id: Ib7b68b577ff903fc2359a8e8c4573d62d55c3828
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>