efi loader: Respect efi_max_resolution in loader.conf(5)

Default the max resolution to 1080p, we'll accept Width x Height
specifications along with the following presets:

• 480p
• 720p
• 1080p
• 2160p or 4k
• 5k

PR: 224825
Differential Revision: https://reviews.freebsd.org/D14801