exynos: add g2d_scale_and_blend

This is a combination of g2d_copy_with_scale and
g2d_scale. I think it's a pretty common operation
to scale one buffer and then blend it on top
of another.
tests/exynos: introduce wait_for_user_input

Currently getchar() is used to pause execution after
each test. The user isn't informed if he's supposed to
do anything for the tests to continue, so just
print a simple message to make this more clear.
exynos: honor the repeat mode in g2d_copy_with_scale

This is useful when the default repeat mode, which is 'repeat'
produces artifacts at the borders of the copied image.
I choose 'pad' mode to use the color of the destination
image.

In my usage case the destination is the framebuffer, which is
solid filled with a background color. Scaling with 'pad' mode
would then just do the right thing and also produces nice
borders on the output.
exynos: introduce g2d_add_base_addr helper function

In almost all functions the base address register is
written, so it makes sense to have a helper function
for this.
exynos: use structure initialization instead of memset
tests/exynos: improve error handling

Check for a usable connector and also if the resolution is sane
(width and height are both non-zero).
exynos: add exynos prefix to fimg2d header
exynos: add fimg2d header to common includes

The reason for this change is to let userspace use this header.
Currently 'make install' does not install the header.
exynos: fimg2d: fix comment for G2D_COEFF_MODE_GB_COLOR

The coefficient mode enables use of global color, not alpha.
exynos: fimg2d: unify register style

Register defines all use uppercase hex codes.
exynos: fimg2d: introduce G2D_OP_INTERPOLATE

Sets up the blending equation in the following way:
out = src * src_alpha + dst * (1 - src_alpha)