backend/drm: get physical size from libdrm

Values from libdrm are likely more reliable than raw values from the EDID. We
were already using values from libdrm, but they were overwritten by parse_edid.
See drm.c:

    wlr_conn->output.phys_width = drm_conn->mmWidth;
    wlr_conn->output.phys_height = drm_conn->mmHeight;

backend/drm/util.c

@@ -118,9 +118,6 @@ void parse_edid(struct wlr_output *restrict output, size_t len, const uint8_t *d
 	uint32_t serial = data[12] | (data[13] << 8) | (data[14] << 8) | (data[15] << 8);
 	snprintf(output->serial, sizeof(output->serial), "0x%08X", serial);
 
-	output->phys_width = ((data[68] & 0xf0) << 4) | data[66];
-	output->phys_height = ((data[68] & 0x0f) << 8) | data[67];
-
 	for (size_t i = 72; i <= 108; i += 18) {
 		uint16_t flag = (data[i] << 8) | data[i + 1];
 		if (flag == 0 && data[i + 3] == 0xFC) {