drm/fb_dma: s/drm_panic_gem_get_scanout_buffer/drm_fb_dma_get_scanout_buffer

On version 11 of the "drm/panic: Add a drm panic handler" series [1], Thomas
suggested to change the name of the function `drm_panic_gem_get_scanout_buffer`
to `drm_fb_dma_get_scanout_buffer` and this request was applied on version 12,
which is the version that landed [2]. Although the name of the function
changed on the C file, it didn't changed on the header file, leading to a
compilation error as such:

drivers/gpu/drm/imx/ipuv3/ipuv3-plane.c:780:24: error: use of undeclared
identifier 'drm_fb_dma_get_scanout_buffer'; did you mean 'drm_panic_gem_get_scanout_buffer'?
  780 |         .get_scanout_buffer = drm_fb_dma_get_scanout_buffer,
      |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                               drm_panic_gem_get_scanout_buffer
./include/drm/drm_fb_dma_helper.h:23:5: note: 'drm_panic_gem_get_scanout_buffer'
declared here
   23 | int drm_panic_gem_get_scanout_buffer(struct drm_plane *plane,
      |     ^
1 error generated.

Fix the compilation error by changing `drm_panic_gem_get_scanout_buffer` to
`drm_fb_dma_get_scanout_buffer` on the header file.

Link: https://lore.kernel.org/dri-devel/20240328120638.468738-1-jfalempe@redhat.com/ [1]
Link: https://lore.kernel.org/dri-devel/aea2aa01-7f03-453b-8b30-8f4d90b1b47f@redhat.com/ [2]
Fixes: 879b3b65

 ("drm/fb_dma: Add generic get_scanout_buffer() for drm_panic")
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240415151013.3210278-1-mcanal@igalia.com