drm: pahole struct drm_display_mode

Reorganize drm_display_mode to eliminate all the holes.
We'll put all the actual timings to the start of the
struct and all the extra junk to the end.

Gets the size down to 136 bytes on 64bit and 120 bytes on
32bit. With a bit more work we should be able to get this
below the two cacheline mark even on 64bit.

v2: Rebase due to DRM_MODE_TYPE_USERDEF comment change

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-11-ville.syrjala@linux.intel.com