video: wire get_metrics for X11 display server, remove from ctx drivers

Wire x11_get_metrics into dispserv_x11.c — the implementation
already existed in gfx/common/x11_common.c (using XDisplayWidthMM,
XDisplayHeightMM, DisplayWidth, DisplayHeight for physical and
pixel dimensions) but was only accessible through context drivers.

Remove x11_get_metrics from the X11 context driver tables:
- x_ctx.c (GLX)
- x_vk_ctx.c (Vulkan)
- xegl_ctx.c (EGL)

Display metrics now route through the display server on X11,
matching the pattern established for Win32, UWP, and Apple.

gfx/display_servers/dispserv_x11.c

@@ -786,7 +786,7 @@ const video_display_server_t dispserv_x11 = {
    NULL, /* get_video_output_size */
    NULL, /* get_video_output_prev */
    NULL, /* get_video_output_next */
-   NULL, /* get_metrics */
+   x11_get_metrics,
    x11_display_server_get_flags,
    "x11"
 };

gfx/drivers_context/x_ctx.c

@@ -1190,7 +1190,7 @@ const gfx_ctx_driver_t gfx_ctx_x = {
    NULL, /* get_video_output_size */
    NULL, /* get_video_output_prev */
    NULL, /* get_video_output_next */
-   x11_get_metrics,
+   NULL, /* get_metrics - handled by display server */
    NULL,
    x11_update_title,
    x11_check_window,

gfx/drivers_context/x_vk_ctx.c

@@ -582,7 +582,7 @@ const gfx_ctx_driver_t gfx_ctx_vk_x = {
    NULL, /* get_video_output_size */
    NULL, /* get_video_output_prev */
    NULL, /* get_video_output_next */
-   x11_get_metrics,
+   NULL, /* get_metrics - handled by display server */
    NULL,
    x11_update_title,
    gfx_ctx_x_vk_check_window,

gfx/drivers_context/xegl_ctx.c

@@ -637,7 +637,7 @@ const gfx_ctx_driver_t gfx_ctx_x_egl =
    NULL, /* get_video_output_size */
    NULL, /* get_video_output_prev */
    NULL, /* get_video_output_next */
-   x11_get_metrics,
+   NULL, /* get_metrics - handled by display server */
    NULL,
    x11_update_title,
    x11_check_window,