exynos/fimg2d: add g2d_validate_xyz() functions

The G2D headers define a number of modes through enums
(like e.g. color, select, repeat, etc.).

This introduces g2d_validate_select_mode() and
g2d_validate_blending_op() which validate a
select mode or blending operation respectively.

Use this together with g2d_check_space() in
g2d_{blend,scale_and_blend}().

For this we move parameter validation to the top and
also validate the select mode of the source image and
the requested blending operation before starting
command submission.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>