Give the gbm- flavours an EGL platform; without one they fall back to
eglGetDisplay() on a GBM device.

Backport of upstream 22c527cb0556 ("Add missing EGL platform for gbm-
flavors"), rebased onto 2023.01, which predates the GLMARK2_USE_NULL
branch the upstream hunk carries as context.

--- a/src/gl-state-egl.cpp
+++ b/src/gl-state-egl.cpp
@@ -479,6 +479,8 @@
 #define GLMARK2_NATIVE_EGL_DISPLAY_ENUM EGL_PLATFORM_WAYLAND_KHR
 #elif  GLMARK2_USE_DRM
 #define GLMARK2_NATIVE_EGL_DISPLAY_ENUM EGL_PLATFORM_GBM_KHR
+#elif  GLMARK2_USE_GBM
+#define GLMARK2_NATIVE_EGL_DISPLAY_ENUM EGL_PLATFORM_GBM_KHR
 #else
 // Platforms not in the above platform enums fall back to eglGetDisplay.
 #define GLMARK2_NATIVE_EGL_DISPLAY_ENUM 0