drm/vkms: Decouple crc operations from composer

In the vkms_composer.c, some of the functions related to CRC and compose
have interdependence between each other. This patch reworks some
functions inside vkms_composer to make crc and composer computation
decoupled.

This patch is preparation work for making vkms able to support new
features.

Tested-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200830142000.146706-2-rodrigosiqueiramelo@gmail.com