drm/sun4i: layers: Use drm_fb_dma_get_gem_addr() to get display memory

drm/sun4i: layers: Use drm_fb_dma_get_gem_addr() to get display memory

Commit 4636ce93d5b2 ("drm/fb-cma-helper: Add drm_fb_cma_get_gem_addr()")
adds a new helper, which covers fetching a drm_framebuffer's GEM object
and calculating the buffer address for a given plane.

This patch uses this helper to replace our own open coded version of the
same function.

Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://patch.msgid.link/20260309165635.1138413-1-wens@kernel.org
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>