drm/i915: Add support for non-power-of-2 FB plane alignment

At least one framebuffer plane on TGL - the UV plane of YUV semiplanar
FBs - requires a non-power-of-2 alignment, so add support for this. This
new alignment restriction applies only to an offset within an FB, so the
GEM buffer itself containing the FB must still be power-of-2 aligned.
Add a check for this (in practice plane 0, since the plane 0 offset must
be 0).

v2:
- Fix WARN check for alignment=0.
v3:
- Return error for alignment programming bugs. (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191231233756.18753-2-imre.deak@intel.com