lima: Fix alpha blending

lima: Fix alpha blending

Introduce separate helper functions to set the blendfactor bits.

Lima uses bits 0-2 for the type, bit 3 sets the inverted function
and bit 4 is set if alpha is used.
alpha_src_factor and alpha_dst_factor don't need the alpha bit, so
they are masked with 0xf. There is only place for 4 bits anyway.
If alpha_src_factor is PIPE_BLENDFACTOR_SRC_ALPHA_SATURATE, we need
to change it to PIPE_BLENDFACTOR_ONE first.
This is exactly what the blob does and we pass all
dEQP-GLES2.functional.fragment_ops.blend.* tests now.
Better than the blob btw...

Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Andreas Baierl <ichgeh@imkreisrum.de>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3411>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3411>