Platforms/RPi3: DisplayDxe virtual resolution improvements

The Pi GPU decouples requested resolution from actual physical resolution
and can perform scaling of virtual resolutions. This enables platform users
to do something like ask for 1024x768 and get a framebuffer of that size,
regardless of the actual output (which could be a very blurry SDTV).

Specifically, this patch allows selecting which specific virtual
resolutions to enable, thus replacing the old all-or-nothing behaviour
with either all virtual resolutions supported, or just the native one.

This patch also adds enables the common 7" Pi (800x480) screen to be used
at 800x600 resolution, instead of forcing 640x480 as the only usable
resolution.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>