loader: Fix 0 vs 1 confusion for column numbers

In two places we use '0' for a column number. However, the upper left
hand corner of the screen is 1, 1. Fix those two confusions. Also, fix
a comment that flipped the coordinates in a comment (I'm used to the
vt100 convention where it's row, column (eg y, x)) and didn't notice
the rest of the code uses x, y.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D46777

(cherry picked from commit 604919c47fa4092db08dadd282851afc9cba0a90)