loader: use display pixel density for font autoselection

Calculate font size from 16 density independent pixels (dp) by using:
size = 16 * ppi/160 * display_factor

We are specifying font size 16dp, and assuming 1dp = 160ppi.
Also apply scaling factor 2 (display_factor).

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28849