drm/hyperv: Fix unused const variable 'hyperv_modifiers'

There is a gcc '-Wunused-const-variable' warning:
  drivers/gpu/drm/hyperv/hyperv_drm_modeset.c:152:23: warning:
    'hyperv_modifiers' defined but not used [-Wunused-const-variable=]

while the variable should be used in drm_simple_display_pipe_init()
as suggested by Thomas, let's fix it.

Fixes: 76c56a5a ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210615031401.231751-1-pulehui@huawei.com