drm/prime: Fix drm_gem_prime_mmap() stack use

drivers/gpu/drm/drm_prime.c: In function 'drm_gem_prime_mmap':
>> drivers/gpu/drm/drm_prime.c:688:1: warning: the frame size of 1592 bytes is larger than 1024 bytes [-Wframe-larger-than=]

Fix by allocating on the heap.

Fixes: 7698799f ("drm/prime: Add drm_gem_prime_mmap()")
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121180215.13881-1-noralf@tronnes.org