drm: Add simpledrm HiDPI display support

drm: Add simpledrm HiDPI display support

For devicetree based systems simpledrm might know the actual physical
display dimensions. Test if the reported connector width and height
result in a pixel density larger than 96 DPI. If that's the case
calculate the device scale instead of guessing it based on the
resolution.
This restores HiDPI scaling on 13-inch Apple silicon Macbooks with a
resolution of 2560x1600 (224 - 227 DPI) after commit 1421a9f6
("ply-utils: Increase threshold for guessed hiDPI scaling
to >= 2880x1620").

Signed-off-by: Janne Grunau <j@jannau.net>