shared/platform: avoid build warning when building without gl-renderer

When building without GL renderer the compiler prints the following
warning:
  ../clients/subsurfaces.c: In function ‘egl_state_create’:
  ../clients/subsurfaces.c:225:35: warning: passing argument 1 of
  ‘weston_platform_get_egl_display’ makes pointer from integer without a
  cast [-Wint-conversion]
    225 |   weston_platform_get_egl_display(EGL_PLATFORM_WAYLAND_KHR,
        |                                   ^~~~~~~~~~~~~~~~~~~~~~~~
        |                                   |
        |                                   int
...

Define the fallback implementation of weston_platform_get_egl_display
to take an integer which is the underlaying datatype of EGLenum.

Signed-off-by: Stefan Agner <stefan@agner.ch>
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