intel: Change the register type from UW to UD in blorp engine

These changes are required to implement scaled blitting in blorp
in my next patch.

No regressions observed in piglit quick-driver.tests with this patch.
intel: Add multisample scaled blitting in blorp engine

In traditional multisampled framebuffer rendering, color samples must be
explicitly resolved via BlitFramebuffer before doing the scaled blitting
of the framebuffer. So, scaled blitting of a multisample framebuffer
takes
two separate calls to BlitFramebuffer.

This patch implements the functionality of doing multisampled scaled
resolve
using just one BlitFramebuffer call. Important changes involved in this
patch are listed below:
    - Use float registers to scale and offset texture coordinates.
    - Change offset computation to consider float coordinates.
    - Round the scaled coordinates down to nearest integer.
    - Modify src texture coordinates clipping to account for scaling..
    - Linear filter is not yet implemented in blorp. So, don't use
      blorp engine to do single sampled scaled blitting.

Note: Observed no piglit regressions on sandybridge & ivybridge with
these changes.
i965: Enable ext_framebuffer_multisample_blit_scaled on intel h/w

This patch enables ext_framebuffer_multisample_blit_scaled extension
on intel h/w >= gen6.

Note: Patches for piglit tests to verify this functionality are out
for review on piglit mailing list. Tests pass for all of the scaling
factors from 0.1 to 2.4.