[fbcon] Remove userptr_t from framebuffer console drivers

[fbcon] Remove userptr_t from framebuffer console drivers

Simplify the framebuffer console drivers by assuming that the raw
framebuffer, character cell array, background picture, and glyph data
are all directly accessible via pointer dereferences.

In particular, this avoids the need to copy each glyph during drawing:
the VESA framebuffer driver can simply return a pointer to the glyph
data stored in the video ROM.

Signed-off-by: Michael Brown <mcb30@ipxe.org>