drm/rockchip: Add missing vmalloc header

The Rockship DRM GEM code uses vmap()/vunmap() so vmalloc header must be
included to avoid warnings like (on IA64, compile tested):

    drivers/gpu/drm/rockchip/rockchip_drm_gem.c: In function ‘rockchip_gem_alloc_iommu’:
    drivers/gpu/drm/rockchip/rockchip_drm_gem.c:134:20: error:
        implicit declaration of function ‘vmap’ [-Werror=implicit-function-declaration]

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1577779956-7612-1-git-send-email-krzk@kernel.org