drm/format-helper: Fix XRGB888 to monochrome conversion

The conversion functions drm_fb_xrgb8888_to_mono() and
drm_fb_gray8_to_mono_line() do not behave correctly when the
horizontal boundaries of the clip rectangle are not multiples of 8:
  a. When x1 % 8 != 0, the calculated pitch is not correct,
  b. When x2 % 8 != 0, the pixel data for the last byte is wrong.

Simplify the code and fix (a) by:
  1. Removing start_offset, and always storing the first pixel in the
     first bit of the monochrome destination buffer.
     Drivers that require the first pixel in a byte to be located at an
     x-coordinate that is a multiple of 8 can always align the clip
     rectangle before calling drm_fb_xrgb8888_to_mono().
     Note that:
       - The ssd130x driver does not need the alignment, as the
	 monochrome buffer is a temporary format,
       - The repaper driver always updates the full screen, so the clip
	 rectangle is always aligned.
  2. Passing the number of pixels to drm_fb_gray8_to_mono_line(),
     instead of the number of bytes, and the number of pixels in the
     last byte.

Fix (b) by explicitly setting the target bit, instead of always setting
bit 7 and shifting the value in each loop iteration.

Remove the bogus pitch check, which operates on bytes instead of pixels,
and triggers when e.g. flashing the cursor on a text console with a font
that is 8 pixels wide.

Drop the confusing comment about scanlines, as a pitch in bytes always
contains a multiple of 8 pixels.

While at it, use the drm_rect_height() helper instead of open-coding the
same operation.

Update the comments accordingly.

Fixes: bcf8b616 ("drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220317081830.1211400-3-geert@linux-m68k.org