parisc/stifb: Implement fb_is_primary_device()

Implement fb_is_primary_device() function, so that fbcon detects if this
framebuffer belongs to the default graphics card which was used to start
the system.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org   # v5.10+