drm/msm/mdp: Add Software Pixel Extension support

In order to produce an image, the scalar needs to be fed extra
pixels. These top/bottom/left/right values depend on a various of
factors, including resolution, scaling type, phase step and
initial phase.

Pixel Extension are programmed by hardware in most targets - and
can be overwritten by software. For some targets (e.g.: msm8996),
software *must* program those registers.

In order to ease this computation, let's always use bilinear
filters, which are easier to program from kernel. Eventually,
all of these values will come down from user space for better
quality.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>