OvmfPkg: PlatformDxe: add form widgets for video modes

OvmfPkg: PlatformDxe: add form widgets for video modes

In this patch we populate the form with the two widgets related to video
resolution:
- A read-only string field displaying the preference for the next boot.
- A drop-down list offering choices for changing the setting. This list is
  implemented with dynamically generated IFR opcodes.

(In general, the current preference may be missing, or it may be invalid
for the available video RAM size. The list of possible new settings is
filtered with the video RAM size.)

Because the form now becomes able to receive input, we must also implement
ExtractConfig(). This function tells the HII engine about the state of the
widgets.

For now we set up both widgets with static data only:
- The current preference always says "Unset". The driver code is still
  isolated from the backend (the UEFI variable store).
- The list of possible resolutions offers 800x600 only. We don't
  interrogate the GOP yet.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15369 6f19259b-4bc3-4df7-8a09-765794883524