drm: increase drm mmap_range size to 1TB

After patch "drm: Use the same mmap-range offset and size for GEM and
TTM", application failed to create bo of system memory because drm
mmap_range size decrease to 64GB from original 1TB. This is not big
enough for applications. Increase the drm mmap_range size to 1TB.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190417221507.933-1-Philip.Yang@amd.com
(cherry picked from commit 96354b5c)