drm/vkms: add alpha-premultiplied color blending

drm/vkms: add alpha-premultiplied color blending

The VKMS blend function was ignoring the alpha channel and just
overwriting vaddr_src with vaddr_dst. This XRGB approach triggers a
warning when running the kms_cursor_crc/cursor-alpha-transparent test
case. In IGT, cairo_format_argb32 uses premultiplied alpha (according to
documentation). Also current DRM assumption is that alpha is
premultiplied. Therefore, this patch considers premultiplied alpha
blending eq to compose vaddr_src with vaddr_dst.

This change removes the following cursor-alpha-transparent warning:

 "Suspicious CRC: All values are 0."

V2:
- static for local functions
- const for the read-only variable argb_src
- replaces variable names
- drops unnecessary comment

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200825114532.abzdooluny2ekzvm@smtp.gmail.com