i965/blorp: Fix clear rectangle alignment in fast color clear

i965/blorp: Fix clear rectangle alignment in fast color clear

From BSpec: 3D-Media-GPGPU Engine > 3D Pipeline > Pixel >
Pixel Backend > MCS Buffer for Render Target(s) [DevIVB+]:
[DevHSW:GT3]: Clear rectangle must be aligned to two times
the number of pixels in the table shown below...
Observed no piglit, gles3conform regressions with this patch.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65744