drm/vkms: Compute CRC without change input data

This commit decouples pixel manipulation from compute_crc() for avoiding
any pixel change during the CRC calculation. This commits represents a
preparation work for making VKMS able to support the writeback feature.

Change in V5 (Melissa):
- Rebase and drop bitmap for alpha
Change in V4 (Emil):
- Move bitmap_clear operation and comments to get_pixel function

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200830142000.146706-3-rodrigosiqueiramelo@gmail.com