graphics: simplify 16bpp calculation
graphics: simplify 16bpp calculation

miguelinux/gummiboot2systemd-boot,liliang365/my_gummiboot,liliang365/my_gummiboot,todorez/tummiboot,freedesktop-unofficial-mirror/gummiboot,miguelinux/gummiboot2systemd-boot,FrozenCow/gummiboot,aaronp24/gummiboot,FrozenCow/gummiboot,todorez/gummiboot-multiboot2,trilean/gummiboot,freedesktop-unofficial-mirror/gummiboot,todorez/tummiboot,aaronp24/gummiboot,todorez/gummiboot-multiboot2,trilean/gummiboot