OMAPDSS: DISPC: Simplify function names for setting pipeline input and output sizes

The DISPC pipeline register names in the TRM for setting the buffer size and
the output size are a bit misleading, for example, there are different register
names for setting the buffer size for VID and GFX pipes. Things get more
confusing when considering writeback pipeline.

Rename the functions so that they tell whether they are configuring the input
to the scalar or the output. These will be extended later to support writeback
registers.

Signed-off-by: Archit Taneja <archit@ti.com>