drm/amdgpu: register a dirty framebuffer callback for fbcon

fbcon requires that we implement &drm_framebuffer_funcs.dirty.
Otherwise, the framebuffer might take a while to flush (which would
manifest as noticeable lag). However, we can't enable this callback for
non-fbcon cases since it may cause too many atomic commits to be made at
once. So, implement amdgpu_dirtyfb() and only enable it for fbcon
framebuffers (we can use the "struct drm_file file" parameter in the
callback to check for this since it is only NULL when called by fbcon,
at least in the mainline kernel) on devices that support atomic KMS.

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: stable@vger.kernel.org # 6.1+
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2519
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>