drm/vmwgfx: Convert macro to octal representation

Change "S_IRUSR | S_IWUSR" to "0600", it's easier to parse mentally.

This change should be part of commit 50f83737 ("drm/vmwgfx: Revert
"drm/vmwgfx: Replace numeric parameter like 0444 with macro""), but the
patch was truncated somewhere in the patch route, so add the missing
change. Extract from the original commit message:

  commit 50f83737
  Author: Øyvind A. Holm <sunny@sunbase.org>
  Date:   Thu Mar 23 14:54:48 2017 -0700

    drm/vmwgfx: Revert "drm/vmwgfx: Replace numeric parameter like 0444
    with macro"

    This reverts commit 2d8e60e8 ("drm/vmwgfx: Replace numeric
    parameter like 0444 with macro")

    The commit belongs to the series of 1285 patches sent to LKML on
    2016-08-02, it changes the representation of file permissions from
    the octal value "0600" to "S_IRUSR | S_IWUSR".

    The general consensus was that the changes does not increase
    readability, quite the opposite; 0600 is easier to parse mentally
    than S_IRUSR | S_IWUSR.

Signed-off-by: Øyvind A. Holm <sunny@sunbase.org>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>