drm/mipi_dbi: Fix off-by-one bugs in mipi_dbi_blank()

When configuring the frame memory window, the last column and row
numbers are written to the column resp. page address registers.  These
numbers are thus one less than the actual window width resp. height.

While this is handled correctly in mipi_dbi_fb_dirty() since commit
03ceb1c8 ("drm/tinydrm: Fix setting of the column/page end
addresses."), it is not in mipi_dbi_blank().  The latter still forgets
to subtract one when calculating the most significant bytes of the
column and row numbers, thus programming wrong values when the display
width or height is a multiple of 256.

Fixes: 02dd95fe ("drm/tinydrm: Add MIPI DBI support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191230130604.31006-1-geert+renesas@glider.be