drm/i915: Align dumb buffer stride to 4k to allow for gtt remapping

Align dumb buffer stride to 4k if the fb will be big enough to
require gtt remapping.

v2: Leave the stride alone for buffers that look to be for the cursor
v3: Make it not a hack (Daniel)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190509122159.24376-7-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>