i965/miptree: Drop an if case from retile_as_linear

Drop an if statement whose predicate never evaluates to true. row_pitch
belongs to a surface with non-linear tiling. According to
isl_calc_tiled_min_row_pitch, the pitch is a multiple of the tile width.
By looking at isl_tiling_get_info, we see that non-linear tilings have
widths greater than or equal to 128B.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit 3df201e3)