OMAPDSS: DISPC: Pipe threshold sizes power optimization.

This patch changes the calculation of thresholds and sets low
threshold to half of buffer size for OMAP4. The memory will
able to sleep between transactions to dss because dss will
read by big chunks.

boype: Also removed dispc_calculate_threshold function, as it is no longer used.

Backport from TI omapzoom kernel 3.4 repo, based upon:
http://git.omapzoom.org/?p=kernel/omap.git;a=commit;h=cd99eb345e1ad26ffbc665f46ea917dad762937b
OMAPDSS: drop small YUV encoded frames on OMAP44xx

OMAP44xx has hardware limitation: in stall mode, when the frame pixel size
is less than output SyncFifo depth(16), DISPC hangs without sending any data.
Simple workaround is to drop such frame, as cases when YUV frames of this size
are very rare.

Change-Id: I31dc262b07a52b66209c093f136ad2bac66f9439
Signed-off-by: Sergiy Kibrik <sergiy.kibrik@globallogic.com>