minigbm: gralloc0: Fix invalid cast from integer to pointer

minigbm: gralloc0: Fix invalid cast from integer to pointer

We should be casting a pointer to the fourcc to a char * pointer,
instead of casting the fourcc value itself (which unluckily happens to
be of the same size as pointer, so compilator fails to detect it).

BUG=b:62358788
TEST=Try to allocate with invalid usage

Change-Id: I2bb94d145dc3765d5193712aad7682c0fb383dd8
Reviewed-on: https://chromium-review.googlesource.com/566791
Commit-Ready: Tomasz Figa <tfiga@chromium.org>
Tested-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>