fbcon: clear the logo bitmap from the margin area

Explicitly clear_margins when clearing the logo, in case the font dimensions
are non-integral to the framebuffer dimensions.

Signed-off-by: Kamal Mostafa <kamal@whence.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>