drm-shim: Support kernels with >4k pages
mmap requires its offset is page aligned, but the current code only guarantees 4k alignment, causing drm-shim to break badly on kernels with >4k page sizes. This fixes drm-shim on my Apple M1, running bare metal Linux with 16k pages. It probably also fixes exotic PowerPC systems with 64k pages. Signed-off-by:Alyssa Rosenzweig <alyssa@collabora.com> Reviewed-by:
Zoltan Boszormenyi <zboszor@gmail.com> Reviewed-by:
Adam Jackson <ajax@redhat.com> Reviewed-by:
Emma Anholt <emma@anholt.net> Cc: mesa-stable Part-of: <!12347> (cherry picked from commit 38f39cc1)