cmake: Correct gbm version check

It appears like gbm_VERSION can be a list, which if() does not like. So
pass the variable to the if() rather than its value. While on this, also
change GREATER_EQUAL to VERSION_GREATER_EQUAL so cmake compares
individual version components.

CMakeLists.txt

@@ -236,12 +236,12 @@ set_package_properties(Libdrm PROPERTIES TYPE REQUIRED PURPOSE "Required for drm
 
 find_package(gbm)
 set_package_properties(gbm PROPERTIES TYPE REQUIRED PURPOSE "Required for egl output of drm backend.")
-if (${gbm_VERSION} GREATER_EQUAL 21.1)
+if (gbm_VERSION VERSION_GREATER_EQUAL 21.1)
     set(HAVE_GBM_BO_GET_FD_FOR_PLANE 1)
 else()
     set(HAVE_GBM_BO_GET_FD_FOR_PLANE 0)
 endif()
-if (${gbm_VERSION} GREATER_EQUAL 21.3)
+if (gbm_VERSION VERSION_GREATER_EQUAL 21.3)
     set(HAVE_GBM_BO_CREATE_WITH_MODIFIERS2 1)
 else()
     set(HAVE_GBM_BO_CREATE_WITH_MODIFIERS2 0)