igt/gem_evict_everything: Compute number of surfaces to saturate the aperture

The goal is to only fill the aperture, not all of memory, so fine-tune
the computed number of surfaces, hopefully avoiding an integer overflow
in the process.

References: https://bugs.freedesktop.org/show_bug.cgi?id=79573
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/tests/eviction_common.c b/tests/eviction_common.c
index f02326b..6c6eab4 100644
--- a/tests/eviction_common.c
+++ b/tests/eviction_common.c
@@ -74,9 +74,9 @@
 	nr_surfaces *= 7;
 	nr_surfaces += 3;
 
-	total_surfaces = (uint64_t)intel_get_total_ram_mb() * 9 /10 * 1024 *1024 / surface_size;
+	total_surfaces = gem_aperture_size(fd) / surface_size + 1;
 	igt_require(nr_surfaces < total_surfaces);
-	igt_require(total_surfaces * surface_size > gem_aperture_size(fd));
+	igt_require(intel_check_memory(total_surfaces, surface_size, CHECK_RAM));
 
 	bo = malloc((nr_surfaces + total_surfaces)*sizeof(*bo));
 	igt_assert(bo);