drm/mipi-dbi: Fix source-buffer address in mipi_dbi_buf_copy

Set the source-buffer address after mapping the buffer into the
kernel's address space. Makes MIPI DBI helpers work again.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: c47160d8 ("drm/mipi-dbi: Remove dependency on GEM CMA helper library")
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reported-by: Noralf Trønnes <noralf@tronnes.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111132634.18302-1-tzimmermann@suse.de