Fixes using uninitialized value error in si_vpe.c

What does this MR do and why?

gallium: Adds return on failure to get plane info

As part of the Help Wanted page for Mesa, I built the project with -Wstrict-aliasing=2 -fstrict-aliasing build flags. Found a warning where the pitch variable at `gallium/drivers/radeonsi/si_vpe.c' is used while uninitialized if the if statement at line 426 fails. Added a return statement with an error code similar to that on line 455.

For testing I ran meson devenv -C build/ glxinfo and it did not show signs of errors. I do not know if running Piglit and/or dEQP is applicable but can run them if required. Most of the meson tests passes save for these two:

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 46/110 test-build-pedantic-staging_cursor_shape_cursor_shape_v1_xml                                                        FAIL            0.08s   exit status 127
>>> MALLOC_PERTURB_=9 ASAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1 UBSAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1:print_stacktrace=1 /home/sassefa/Code/mesa/build/subprojects/wayland-protocols-1.32/tests/test-build-pedantic-staging_cursor_shape_cursor_shape_v1_xml

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107/110 test-build-pedantic-unstable_xdg_decoration_xdg_decoration_unstable_v1_xml                                          FAIL            0.07s   exit status 127
>>> MALLOC_PERTURB_=111 ASAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1 UBSAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1:print_stacktrace=1 /home/sassefa/Code/mesa/build/subprojects/wayland-protocols-1.32/tests/test-build-pedantic-unstable_xdg_decoration_xdg_decoration_unstable_v1_xml

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Ok:                 108 
Expected Fail:      0   
Fail:               2   
Unexpected Pass:    0   
Skipped:            0   
Timeout:            0