i965: Examine _BaseFormat when deciding to perform xRGB_alpha fixups

The renderbuffer's Format field may have an alpha channel even when the
underlying _BaseFormat does not. This can happen when mesa chooses to use
RGBA16 for an RGB16 format, for example.

So look at _BaseFormat when deciding whether to fixup the blend factors.

This test improves the results of at least the following piglit tests:

	EXT_frambebuffer_object/fbo-blending-formats
        	{GL_RGB10, GL_RGB12, GL_RGB16}
	EXT_texture_snorm/fbo-blending-formats
		{GL_RGB16_SNORM, GLRGB8_SNORM, GL_RGB_SNORM}

But none of these actually change from FAIL to PASS yet. The R, G, and B probe
values are fixed with this commit, but the tests still fail because the alpha
values are still wrong.

Reviewed-by: Eric Anholt <[email protected]>

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