drm/edid: Fix uninitialized variable in drm_cvt_modes()

Noticed this when trying to compile with -Wall on a kernel fork. We
potentially don't set width here, which causes the compiler to complain
about width potentially being uninitialized in drm_cvt_modes(). So, let's
fix that.

Changes since v1:
* Don't emit an error as this code isn't reachable, just mark it as such
Changes since v2:
* Remove now unused variable

Fixes: 3f649ab728cd ("treewide: Remove uninitialized_var() usage")
Signed-off-by: Lyude Paul <[e-mail]>
Reviewed-by: Ilia Mirkin <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_edid.c

···
 		case 0x0c:
 			width = height * 15 / 9;
 			break;
+		default:
+			unreachable();
 		}
 
 		for (j = 1; j < 5; j++) {