MFC fbd(4) and vt_fb disentanglement:

r268472 (ray):

  Should check fb_read method presence instead of double check for fb_write.

r269620 (nwhitehorn):

  Retire various intertwined bits of fbd(4) and vt_fb, in particular the
  pixel modification indirection. No actual drivers use it and those
  that might (e.g. creatorfb) use custom implementations of
  vd_bitbltchr().

Relnotes:	No
Sponsored by:	The FreeBSD Foundation
MFC r269636 by nwhitehorn:

  Set fb_pbase properly on PowerPC in the case where we have to guess at
  the right register bank for the framebuffer. Disable the assigned-
  addresses path on SPARC since it is just a hack for IBM PPC systems
  and was neither relevant for nor worked on SPARC anyway.

Sponsored by:	The FreeBSD Foundation
MFC r269685 (nwhitehorn): Retire vd_maskbitbltchr.

  The same functionality can be obtained by testing for mask != NULL in
  vd_bitbltchr, which all implementations of vd_bitbltchr() were doing
  anyway.

Sponsored by:	The FreeBSD Foundation