i915: fallback for NPOT cubemap texture

i915: fallback for NPOT cubemap texture

Although some hardware support NPOT cubemap, but it seems we don't know
the right layout for NPOT cubemap. Thus seems we need do fallback for
other platforms as well.

See comments inline the code for more detailed info.

v2: give a more detailed info about why we need fallback for other
    platfroms as well.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=46666

NOTE: This is a candidate for stable release branches.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>