tgafb: avoid restriction on modes with line length not a multiple of 64

In tgafb there is a restriction that prevents the user from setting a
videomode with line length not a multiple of 64 bytes (for example,
800x600 is not allowed).

The reason for this restriction it that functions copyarea_line_8bpp and
copyarea_line_32bpp can not handle a line length that is not a multiple
of 64 bytes.

This patch removes this restriction on mode setting and makes sure that
the functions copyarea_line_8bpp and copyarea_line_32bpp are called only
if line length is a multiple of 64 bytes. If we set a mode 800x600,
the functions copyarea_line_8bpp and copyarea_line_32bpp are not used,
generic functions for copying are used instead and it works just fine.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>