Fonts: Add charcount field to font_desc

Subsystems are hard-coding the number of characters of our built-in fonts
as 256. Include that information in our kernel font descriptor, `struct
font_desc`.

Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/65952296d1d9486093bd955d1536f7dcd11112c6.1605169912.git.yepeilin.cs@gmail.com