pvr: fix typo in pvr_rt_get_region_headers_stride_size()

pvr: fix typo in pvr_rt_get_region_headers_stride_size()

This function was returning the size of a single region header as the stride
when it was supposed to be returning the total size of the region headers for a
single render target. This went unnoticed due to the fact this function had two
variables with basically identical names. To avoid any future confusion, rename
rgn_header_size to single_rgn_header_size throughout the code.

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22574>