gallivm: Silence uninitialized variable warnings.

gallivm: Silence uninitialized variable warnings.

Fixes these GCC warnings.
gallivm/lp_bld_sample_aos.c: In function 'lp_build_sample_image_linear':
gallivm/lp_bld_sample_aos.c:439: warning: 'r_ipart' may be used uninitialized in this function
gallivm/lp_bld_sample_aos.c:438: warning: 't_ipart' may be used uninitialized in this function
gallivm/lp_bld_sample_aos.c:438: warning: 't_fpart' may be used uninitialized in this function
gallivm/lp_bld_sample_aos.c:439: warning: 'r_fpart' may be used uninitialized in this function
gallivm/lp_bld_sample_aos.c:438: warning: 't_fpart_hi' may be used uninitialized in this function
gallivm/lp_bld_sample_aos.c:438: warning: 't_fpart_lo' may be used uninitialized in this function
gallivm/lp_bld_sample_aos.c:439: warning: 'r_fpart_hi' may be used uninitialized in this function
gallivm/lp_bld_sample_aos.c:439: warning: 'r_fpart_lo' may be used uninitialized in this function