mesa/egl: Resolve GCC sign-compare warning.

mesa/egl: Resolve GCC sign-compare warning.

mesa/src/egl/drivers/dri2/platform_drm.c: In function 'release_buffer':
mesa/src/egl/drivers/dri2/platform_drm.c:73:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++) {
                  ^
mesa/src/egl/drivers/dri2/platform_drm.c: In function 'has_free_buffers':
mesa/src/egl/drivers/dri2/platform_drm.c:87:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++)
                  ^
mesa/src/egl/drivers/dri2/platform_drm.c: In function 'dri2_drm_destroy_surface':
mesa/src/egl/drivers/dri2/platform_drm.c:199:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++) {
                  ^
mesa/src/egl/drivers/dri2/platform_drm.c: In function 'get_back_bo':
mesa/src/egl/drivers/dri2/platform_drm.c:224:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++) {
                     ^
mesa/src/egl/drivers/dri2/platform_drm.c: In function 'dri2_drm_swap_buffers':
mesa/src/egl/drivers/dri2/platform_drm.c:425:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
          for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++)
                        ^

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>