drm/mgag200: Remove declaration of mgag200_mmap() from header file

Commit 94668ac7

 ("drm/mgag200: Convert mgag200 driver to VRAM MM")
removed the implementation of mgag200_mmap(). Also remove the declaration.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Fixes: 94668ac7 ("drm/mgag200: Convert mgag200 driver to VRAM MM")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Armijn Hemel <armijn@tjaldur.nl>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: <stable@vger.kernel.org> # v5.3+
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-2-tzimmermann@suse.de