drm/st7586: Call MIPI DBI mode_valid helper

MIPI DBI drivers validate each mode against their native resolution.
Add this test to st7586.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Noralf Trønnes <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/tiny/st7586.c

···
 };
 
 static const struct drm_simple_display_pipe_funcs st7586_pipe_funcs = {
+	.mode_valid	= mipi_dbi_pipe_mode_valid,
 	.enable		= st7586_pipe_enable,
 	.disable	= st7586_pipe_disable,
 	.update		= st7586_pipe_update,