drm-formats: add weston_drm_format_array_count_pairs()

It simply returns the number of format/modifier pairs in the array. This
will be useful for the next commits, in which we add support for dma-buf
feedback.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>

tests/drm-formats-test.c

@@ -81,6 +81,8 @@ TEST(basic_operations)
 
         weston_drm_format_array_init(&format_array);
 
+        assert(weston_drm_format_array_count_pairs(&format_array) == 0);
+
         ADD_FORMATS_AND_MODS(&format_array, formats, modifiers);
 
         for (i = 0; i < ARRAY_LENGTH(formats); i++) {
@@ -90,6 +92,9 @@ TEST(basic_operations)
                         assert(weston_drm_format_has_modifier(fmt, modifiers[j]));
         }
 
+        assert(weston_drm_format_array_count_pairs(&format_array) ==
+               ARRAY_LENGTH(formats) * ARRAY_LENGTH(modifiers));
+
         weston_drm_format_array_fini(&format_array);
 }