drm/modes: Only consider bpp and refresh before options

Some video= options might have a value that contains a dash. However, the
command line parsing mode considers all dashes as the separator between the
mode and the bpp count.

Let's rework the parsing code a bit to only consider a dash as the bpp
separator if it before a comma, the options separator.

A follow-up patch will add a unit-test for this once such an option is
introduced.
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v4-12-60d38873f782@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/drm_modes.c

@@ -1801,20 +1801,22 @@ bool drm_mode_parse_command_line_for_connector(const char *mode_option,
 
 	name = mode_option;
 
+	/* Locate the start of named options */
+	options_ptr = strchr(name, ',');
+	if (options_ptr)
+		options_off = options_ptr - name;
+	else
+		options_off = strlen(name);
+
 	/* Try to locate the bpp and refresh specifiers, if any */
-	bpp_ptr = strchr(name, '-');
+	bpp_ptr = strnchr(name, options_off, '-');
 	if (bpp_ptr)
 		bpp_off = bpp_ptr - name;
 
-	refresh_ptr = strchr(name, '@');
+	refresh_ptr = strnchr(name, options_off, '@');
 	if (refresh_ptr)
 		refresh_off = refresh_ptr - name;
 
-	/* Locate the start of named options */
-	options_ptr = strchr(name, ',');
-	if (options_ptr)
-		options_off = options_ptr - name;
-
 	/* Locate the end of the name / resolution, and parse it */
 	if (bpp_ptr) {
 		mode_end = bpp_off;