OMAPDSS: Add support for MFLAG

OMAP5 has support for MFLAG feature, which allows DSS to dynamically
increase the priority of DISPC's DMA traffic. At the moment we don't
have support for it.

It was noticed that on DRA7 with high bandwidth use cases we see FIFO
underflows. Implementing MFLAG support removed those underflows.
Interestingly, on OMAP5 uEVM no such overflows were seen.

This patch adds a simple MFLAG implementation, where we use a fixed
MFLAG threshold value based on the FIFO size. The thresholds are set to
4/8 of fifo size for low threshold, and 5/8 of fifo size for high
threshold.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>